The AirGun Guild

General Category => Big Bore Airguns => Topic started by: rsterne on December 04, 2019, 03:12:27 PM

Title: My .257 Condor Build
Post by: rsterne on December 04, 2019, 03:12:27 PM
I got a screaming deal on a good used .22 cal Condor a month ago, c/w a Hawke 6-24 x 50 SF Scope and BiPod…. I have never had one before, so it will be a bit of a learning experience.... As it turns out, I also bought a .257 cal 14" twist TJ's barrel that was 1/2" diameter and 26" long, which I didn't really have a home for, and it hadn't even arrived when I bought the gun.... It was deserving of a good home, so I decided to convert the Condor to .257 cal, it just seems a better fit than a .22 cal in such a powerful platform....

Lloyd Sikes offered me a piece of 14mm ID x 23mm OD carbon fibre tubing for a barrel sleeve, so all I had to do was shim the barrel up to 14mm.... I had a piece of 1/2" ID x 9/16" OD carbon tubing here left over from my Model Yachting days, so I glued that to the barrel and then sanded it down to fit inside the large 14mm ID tube.... I also got a short piece of .45 cal barrel that was 5/8" OD from Lloyd to use as a shim for the part behind the collars that the hammer slides on, and drilled it out to 1/2" ID to slide over the barrel, and Loctite'd it in place.... I made two new collars from 6061-T6 aluminum, turned to fit inside the Condor tube (1.010" OD)…. The front one was drilled out 7/8" ID and the rear one was 1" long, with the rear half drilled to 5/8" like the Condor collar.... but a recess in the front drilled 7/8" ID by 1/2" deep.... I then turned the rear part of the 23mm OD carbon tubing to fit inside the collars, and ended up with these parts....

(https://i378.photobucket.com/albums/oo221/rsterne/257%20Condor/257%20Condor%20Barrel%20Parts_zps5bvuos5o.jpg) (http://s378.photobucket.com/user/rsterne/media/257%20Condor/257%20Condor%20Barrel%20Parts_zps5bvuos5o.jpg.html)

The back inch of the 23mm CF tube is drilled to 5/8" to slide over the CrMoly tube made from the .45 cal barrel.... In this way, the end of that 5/8" steel tube stops inside the CF tube, and the OD of the tube fits inside the rear collar.... so nothing ends at the same point to spread out any possible bending loads in that area.... The thin front collar is just glued to the CF tube, but it will be screwed to it after the final barrel assembly and all the glue dries.... All gluing is done with Loctite 638, and after it dries overnight, it will get a 3 hour post-cure in my wife's over at 175*F.... I'll have a photo of the assembled .257 cal barrel tomorrow....

Bob
Title: Re: My .257 Condor Build
Post by: steveoh on December 04, 2019, 05:42:40 PM
Now this is exciting! Love hearing about .257’s and wrapping the barrels in carbon fiber.

I saw that Travis is finally working on .257 Raptor builds. Will be interesting to see how they go.
Title: Re: My .257 Condor Build
Post by: sixshootertexan on December 04, 2019, 09:01:24 PM
Awesome Bob. I been wanting to build a AF .257 but I have other things going right now.
Title: Re: My .257 Condor Build
Post by: rsterne on December 05, 2019, 06:22:29 PM
Here is the completed .257 barrel, along with the stock .22 cal Condor barrel....

(https://i378.photobucket.com/albums/oo221/rsterne/257%20Condor/Condor%20Barrels_zps0gatbegj.jpg) (http://s378.photobucket.com/user/rsterne/media/257%20Condor/Condor%20Barrels_zps0gatbegj.jpg.html)

The collars are glued on with Loctite 638, along with the CF sleeve and 5/8" CrMoly sleeve in the hammer/spring portion.... The muzzle is threaded 1/2"-20NF to accept a Hatsan Air Stripper or a shot-filled damper.... There is an O-ring on the barrel that pushes into the gap at the front of the Condor main tube.... basically just to keep any dirt out.... The barrel, sleeved with a 23mm OD carbon fibre tube, is about 4 times as stiff as the stock 5/8" barrel.... and yet the assembly is 1 oz. lighter, even though the new barrel is 2" longer....  8)

As in the stock barrel, the four 10-32 set screws that locate the barrel inside the Condor tube tighten against the barrel.... The assembly should be VERY rigid....   ;) …. Here is the barrel installed in the frame....

(https://i378.photobucket.com/albums/oo221/rsterne/257%20Condor/Condor%20257%20Barrel_zpstl8mvz7q.jpg) (http://s378.photobucket.com/user/rsterne/media/257%20Condor/Condor%20257%20Barrel_zpstl8mvz7q.jpg.html)

Bob
Title: Re: My .257 Condor Build
Post by: sixshootertexan on December 06, 2019, 10:02:20 AM
I'd love to see what this bullet would do.
257-112-FN 2 cavity BT
Title: Re: My .257 Condor Build
Post by: steveoh on December 06, 2019, 10:14:40 AM
I'd love to see what this bullet would do.
257-112-FN 2 cavity BT

From my experience it looks a bit heavy.
Title: Re: My .257 Condor Build
Post by: rsterne on December 06, 2019, 03:32:01 PM
That is certainly way too heavy for the stock valve.... It would take about 4000 psi or maybe more in the 26" barrel to hit the 900s, with a wide open valve, even in a Condor.... My Monocoque .257 can just push them at 950, running wide open on 4000 psi, with a 33" barrel.... Detuned for reasonable efficiency they shoot at 900....

Here is the assembled gun with the .257 barrel, but everything else stock....

(https://i378.photobucket.com/albums/oo221/rsterne/257%20Condor/257%20Condor%20Email_zpsochmevid.jpg) (http://s378.photobucket.com/user/rsterne/media/257%20Condor/257%20Condor%20Email_zpsochmevid.jpg.html)

Now bear in mind I have never shot a Condor before, so I have no idea what to expect or how to tune them.... I made a guess that with the stock valve the 63.4 gr. BBT FNs I have would be about the right weight, and it was a good guess.... Here is a chart of the velocity at all power wheel settings....

(https://i378.photobucket.com/albums/oo221/rsterne/257%20Condor/257%20Condor%20Stock%20PW_zpsbci9evky.jpg) (http://s378.photobucket.com/user/rsterne/media/257%20Condor/257%20Condor%20Stock%20PW_zpsbci9evky.jpg.html)

What I didn't expect, and don't understand, is why the velocity decreases, and the report is reduced, at PW 9 and above....  ??? …. Is the hammer driving the breech and top hat so hard it is bouncing off the valve and reducing the dwell?.... Most PCPs simply hit the plateau velocity and then sit there.... In any case, the gun maxed out at 915 fps (118 FPE) with the stock valve, at 3000 psi.... not too bad, I think....  I then ran some strings at various PW settings, with the following results....

(https://i378.photobucket.com/albums/oo221/rsterne/257%20Condor/257%20Condor%20Stock%20Strings_zpsjvat9edq.jpg) (http://s378.photobucket.com/user/rsterne/media/257%20Condor/257%20Condor%20Stock%20Strings_zpsjvat9edq.jpg.html)

At PW 9 the velocities were all over the place, so I didn't include them.... At PW 8 I got only 3 declining shots within a 4% ES, and the efficiency was about 0.86 FPE/CI.... At PW 7 that increased to 4 shots within 4% at 0.93 FPE/CI.... When I reduced the preload to PW 6, the number of shots within 4% increased to 6, at 0.95 FPE/CI, but the velocity was still declining every shot....

A bell-curve developed at PW 5, giving me 9 shots within a 4% ES at an average of 881 fps (109 FPE) and an efficiency of 1.08 FPE/CI.... At PW 4, the number of shots within 4% increased to 12 (the first shot at 3000 psi was too slow), with the average velocity down to 869 fps (106 FPE) at 1.10 FPE/CI.... It looks like a setting between PW 4 and PW 5 would give me 10 shots within a 4% ES, starting at 3000 psi and ending at about 2500 psi....

This gives me a good baseline for my .257 Condor.... As a comparison, my .257 Hayabusa, with its 460 cc reservoir filled to 3000 psi and using a 66 gr. slug, gets 11 shots within 4% averaging 938 fps (129 FPE) ending at 2340 psi, which is 1.11 FPE/CI.... However, it has a 28" barrel and the smallest port is the equivalent of 0.241".... The Hayabusa peaks at 160 FPE with an 88 gr. slug.... To get more power, the Condor will need larger ports (or increased pressure)….

Bob
Title: Re: My .257 Condor Build
Post by: steveoh on December 07, 2019, 09:29:27 AM
My Texan with the give or take 72 grain 257420 or the Noe BT of similar weight likes the power wheel set pretty low. I can get a bell curve and about 10 shots. I do wish AirForce had put a numerical scale on the power wheel for my own reference and not for pissing contests.
Title: Re: My .257 Condor Build
Post by: rsterne on December 07, 2019, 03:28:53 PM
I have been trying to wrap my brain around the forces on the Condor valve, in particular when the valve is open.... When the valve is closed, it has HPA inside the tank, and atmospheric pressure inside the stem, breech and barrel.... so the force holding the valve closed is the seat area of the poppet times the pressure, just like any other PCP.... However, when the valve is open, but before the slug leaves the muzzle, it is completely different than a conventional PCP....

In a conventional PCP, when the valve is open, there are three closing forces on the valve.... In order of importance:

1. The pressure in the valve throat times the area of the stem that penetrates the valve body (with atmospheric pressure outside)….

2. The drag (or pressure differential) across the head of the poppet, caused by the airflow past it.... This starts out low, and increases as the slug accelerates down the bore and the airflow through the valve increases....

3. The valve spring....

In the Condor (and Texan) valve, however, the end of the stem is not exposed to atmospheric pressure as long as the slug/pellet is in the barrel.... Inside the sliding breech, the pressure is very nearly the same as inside the tank.... This means that there is a force on the top hat (which seals the back of the breech) acting to hold the valve OPEN....  :o …. Here is a sketch I made to show what I mean....

(https://i378.photobucket.com/albums/oo221/rsterne/257%20Condor/Condor%20Pressures_zpssxqibxu7.jpg) (http://s378.photobucket.com/user/rsterne/media/257%20Condor/Condor%20Pressures_zpssxqibxu7.jpg.html)

The Breech and Valve Body are in blue, and the Poppet Head, Valve Stem and Top Hat are in black.... The valve is shown open, and I am assuming that the pressure inside the tank, valve stem, and breech is the same everywhere.... This means that the head of the poppet is surrounded by HPA, and there is no force on it in any direction (neglecting airflow for the moment)…. The forces, represented by the teal coloured lines, cancel out.... The valve stem is sealed in the valve body with an O-ring, through which it slides.... If the OD of the stem is a constant, it doesn't matter how big the hole through it is, because the exposed area on both ends (represented by the green lines) also cancels out... This leaves the Top Hat.... The front end of it, which seals in the breech, is larger than the valve stem, where it seals inside the valve body.... This means that a force is generated, represented by the red lines, trying to push the valve OPEN....  :o

If the OD of the stem where it slides through the O-ring in the valve body is "d" and the OD of the top hat is "D", we can calculate that force as follows:

[ (D x D) - (d x d) ] x PI/4 x (Pressure inside the Breech)

For my stock Condor valve, at 3000 psi, I get.... [ (0.309 x 0.309) - (0.246 x 0.246) ] x PI/4 = 0.0274 x 3000 = 82 lbs....  :o

That seems like a very high number, when you consider that the valve spring is only a few lbs. force.... so the fact that the valve closes at all must be due to the drag of the air moving past the poppet and through the valve stem, and the pressure differential that causes between the tank and inside the breech.... That not only reduces the opening force on the top hat area, but adds a closing force as well.... Obviously, this must work, or the valve would always stay open until the slug exits the muzzle and the pressure in the breech drops drastically....

This force, which for a given valve stem OD is dependent on the ID of the breech and the pressure inside it, explains some things about the infamous Condor "tank dump".... For the tank to dump, it seems reasonable to me that this opening force on the top hat must still be large enough to hold the valve open even after the slug has left the muzzle.... That means that the pressure inside the breech cannot escape fast enough through the open bore.... The larger the OD of the top hat (which must be the same as the OD of the barrel stub), the greater the opening force.... Since that OD is the same as the OD of the barrel stub that slides inside the breech, a thicker wall in that area will trap more pressure and increase the likelihood of a dump.... This is because the air cannot escape out of the muzzle fast enough to drop the pressure in the breech, and that increases the force on the top hat, and that overcomes the closing forces on the valve....

Additionally, the opening force on the top hat assists the hammer in getting enough dwell to produce the desired power.... For a given top hat (and barrel) OD, the larger the caliber, the more work the hammer must do to create the shot, because the barrel volume is larger, and as the slug moves down the bore, the greater the pressure drop in the breech, and the less assist to the hammer from the opening force on the top hat.... The opposite occurs with a heavy slug.... It moves slower down the barrel, increasing the pressure inside the breech, which increases the force on the top hat, and increasing the chance of a tank dump....

Assuming my thoughts are correct (and this is only my first kick at the can)…. I think a lot of the variables in tuning a Condor have to do with the relationship between the caliber and the OD of the barrel stub.... and how freely the air can flow through the valve stem.... The bigger and less restrictive the ports in the stem, the less the pressure differential should be (particularly near the end of the shot) and the higher the breech pressure.... This reduces the closing force on the poppet, and should increase the chance of a tank dump....

I look forward to our Condor experts reading through this post and commenting.... I may have it completely wrong.... and if so, I want to know where I made my mistakes....  8)

Bob
Title: Re: My .257 Condor Build
Post by: rsterne on December 11, 2019, 01:45:31 PM
Today I tried installing an O-ring between the Top Hat and the front of the valve.... I had a few thicknesses to choose from, and started with a standard # 010, which is 1/4" ID with a 0.070" Cross Section.... The gun had about 2700 psi in it, so I set the Power Wheel at 5, which if the O-ring was not being hit by the TH should have given me a velocity of about 880 fps, based on the tests I ran with no O-ring.... The velocity was only 760 fps, so obviously that O-ring was much too thick for my .257 Condor.... I removed that O-ring and installed the thinnest one I had instead.... This was a 6 mm ID with only a 1 mm CS.... At the same pressure, the velocity was 872 fps, very close to what I got at the same pressure without any O-ring, so I refilled the gun to 3000 psi and shot a string until the velocity fell more than 4% below the peak.... I then repeated that procedure, but with the PW set at 6 instead of 5.... The results, along with those from the initial tests with no O-ring, are below.... The black lines are at PW = 6, while the red lines are at PW = 5.... The blue line is without O-ring and PW = 4.... The solid lines are without the O-ring and the dashed lines are with the 1 mm O-ring installed.... All lines only show shots within a 4% ES....

(https://i378.photobucket.com/albums/oo221/rsterne/257%20Condor/257%20Condor%20O-ring%20Strings_zpsu5n4linj.jpg) (http://s378.photobucket.com/user/rsterne/media/257%20Condor/257%20Condor%20O-ring%20Strings_zpsu5n4linj.jpg.html)

There are three obvious differences with the O-ring installed.... Firstly, the peak velocity is about 10 fps less than the same PW setting without the O-ring.... Secondly, the shot count is significantly increased.... Thirdly, the shot strings are flatter (not as peaky), and in fact whereas at PW 6 without the O-ring I had a declining shot string, with the 1 mm O-ring installed I had a bell-curve, although barely.... What you can't see from the chart is that the efficiency also improved.... Here are the average results for the 5 shot strings above....

PW = 6.... 6 shots averaging 879 fps (108.8 FPE) at 0.95 FPE/CI
PW = 6 with O-ring.... 10 shots averaging 880 fps (109.0 FPE) at 1.02 FPE/CI
PW = 5.... 9 shots averaging 881 fps (109.2 FPE) at 1.08 FPE/CI
PW = 5 with O-ring.... 12 shots averaging 872 fps (107.2 FPE) at 1.10 FPE/CI
PW = 4.... 12 shots averaging 869 fps (106.2 FPE) at 1.10 FPE/CI

Just out of curiousity, I also shot 5 shots with an 82.5 gr. slug, with the O-ring in place, a 3000 psi fill and the PW at 6.... I got a declining shot string starting at 3000 psi, averaging 822 fps (124 FPE)…. I also wanted to see what effect the O-ring had on the Power Wheel settings, so I checked the velocity at a 3000 psi fill, with the following results....

(https://i378.photobucket.com/albums/oo221/rsterne/257%20Condor/257%20Condor%20O-ring%20PW_zpsdrofstpp.jpg) (http://s378.photobucket.com/user/rsterne/media/257%20Condor/257%20Condor%20O-ring%20PW_zpsdrofstpp.jpg.html)

As you can see, the O-ring started having an effect on the velocity at PW = 6, initially reducing it slightly as the PW setting was increased.... However, at PW = 9 and above, where the hammer was previously bouncing back and the velocity was reduced.... with the O-ring in place the hammer was slowed more gradually, and the velocity peaked at PW = 11 at 930 fps, instead of at PW = 8 at 915 fps.... The O-ring cushioned the contact between the TH and the front of the valve, and produced a much more normal " velocity plateau"....

In this case, the commonly available American sized O-ring, with a 0.070" CS, was much too thick for my .257 Condor.... I might work fine in a .22 cal where you wanted to detune it and extend the shot string, but all it did was lose me a lot of power.... However, the 1 mm CS O-ring was just about perfect for this gun.... It requires the PW setting to be increased by 1 to get the same velocity, but extends the shot string and increases the efficiency at the same time.... What's not to like.... !?!?!

Bob
Title: Re: My .257 Condor Build
Post by: rsterne on December 12, 2019, 10:54:25 AM
For those of you not familiar with adding an O-ring to the valve stem on a Condor, behind the top hat, here is a photo of the tank with it installed.... You don't need to remove the top hat, just stretch the O-ring over it to install it....

(https://i378.photobucket.com/albums/oo221/rsterne/257%20Condor/Tank%20and%20O-rings_zpssrzteifj.jpg) (http://s378.photobucket.com/user/rsterne/media/257%20Condor/Tank%20and%20O-rings_zpssrzteifj.jpg.html)

Below the valve is a 6mm ID x 1mm CS O-ring (left) beside a standard 1/4" ID x 0.070" CS O-ring to show you how much thinner the 1mm is.... The 0.070" thick O-ring severely decreased the power on my .257 Condor because it limited the valve lift (and dwell) too much....

Bob
Title: Re: My .257 Condor Build
Post by: steveoh on December 12, 2019, 12:15:31 PM
My Texan has an O-Ring there. Hmmm. I wonder if that’s stock?
Title: Re: My .257 Condor Build
Post by: rsterne on January 29, 2020, 03:04:18 PM
Today I made a degassing tool for the Condor.... It threads onto the valve body and has a stem that can be screwed in to depress the Top Hat to vent the valve....

(https://i378.photobucket.com/albums/oo221/rsterne/257%20Condor/Degasser%20Parts%20Small_zpsnmqz5ajm.jpg) (https://s378.photobucket.com/user/rsterne/media/257%20Condor/Degasser%20Parts%20Small_zpsnmqz5ajm.jpg.html)

The body is a piece of 1.25" OD 6061-T6 aluminum bar stock, 2" long.... It is drilled in 1.5" with a 13/16" drill and tapped 7/8"-14 NF to match the valve body.... Then the deeper half of the hole was bored out to 0.88", a bit larger than the threads to form a cavity slightly larger than the thread OD.... It is counterbored 7/8" for 0.10" depth to allow it to screw onto the valve right against the shoulder.... Two 3/32" holes are drilled on a 5 deg. angle in the lip that come out in the cavity just beyond the threads.... and notches filed to vent those holes at the valve body.... The stem that presses on the top hat is made from a 1" long 7/16"-20 NF hex head bolt with the head turned flat and round, and drilled with a size "P" drill (0.323") to a depth of 0.40", and crossdrilled with a 3/32" hole to vent the air from the inside of the bolt to the chamber, where it can then vent out at the collar of the valve.... I cut a screwdriver slot in the end of the bolt so that it can be screwed in against the top hat.... Here is it installed on the valve....

(https://i378.photobucket.com/albums/oo221/rsterne/257%20Condor/Condor%20Degasser%20Small_zpsd5uoyote.jpg) (https://s378.photobucket.com/user/rsterne/media/257%20Condor/Condor%20Degasser%20Small_zpsd5uoyote.jpg.html)

To use it, you back off the stem all the way, and then thread the body onto the valve and tighten it by hand against the shoulder.... You then slowly tighten the stem until you feel it touch the top hat.... A slight amount more pressure and the air in the tank vents out through the degasser, coming out through the slots against the valve shoulder.... Make SURE you don't have your hand near those slots, as the escaping HPA could puncture your skin, giving you an embolism.... As the tank pressure drops, the rate of the air venting slows, and you may have to screw the stem in a bit more.... The tank, and the degassing tool, will get VERY cold as the air vents out.... Once all the air is out of the tank, you can remove the degassing tool so that you can work on the valve....

Now, if somebody will tell me how to remove the valve, I would be grateful…. I have a steel bar that fits accurately in the hole in the valve collar opposite the burst disc, but even with quite a bit of pressure, the valve won't budge.... I don't want to force it, and I assume it is Loctited in place, as there seems to be some glue residue where the valve meets the tank.... I'm guessing that I need to heat the tank, will a heat gun do the job?.... More importantly, is there something I missed?....  ???

Bob
Title: Re: My .257 Condor Build
Post by: rsterne on January 30, 2020, 03:02:20 PM
I managed to get the valve off with some heat from a heat gun.... Once it softened the glue and it moved, after that it spun off no problem.... I had a bit of trouble figuring out how to remove the top hat and stem.... The top hat threads onto the stem from the back (bottom side, against the valve), so can't be removed without pulling the stem out of the valve.... You do that by unscrewing the brass collar with the two slots in it to remove the valve assembly from the valve body.... Remove the bottom brass part first, to remove the spring, as that reduces the load on the front collar and it spins out easily....

Once I had the inner valve assembly out of the valve body, I turned the top hat in towards the valve until it touched the 1mm O-ring I had there.... I then used a screwdriver between the top hat and the brass collar to pry upwards a bit, which starts the other end of the stem pulling out of the Delrin poppet head.... Once it moved a bit, I turned the top hat down again, to reduce the gap the screwdriver was in (to keep it working easily) and repeated this operation until the stem pulled out of the poppet.... There were no splines or grooves in my stem, so it wasn't too hard to remove....

Once I had the valve all apart, I had a good look at the air passage in the stem.... The front part is drilled out 0.215", and the rear is crossdrilled with a 5/32" drill from each side to meet almost in the middle.... The air has to make two nearly 90 deg. turns, plus the back edge of the cross holes is shrouded by the conical part of the stem, and the brass valve body when the valve opens.... It is those two holes that feed the inside of the stem I wanted to work on.... I had seen some stems where the 1/4" OD part of the stem was simply milled away on each side, but I had also been warned that if you weaken the stem too much in that area, it will bend or break.... I decided I would use a 5/32" mill on an angle to ease the flow but leave as much metal as possible where the conical part of the stem meets the small diameter area behind that....

The trick was trying to figure out how to hold the stem.... The front part of is it threaded for the top hat, and is larger than 1/4".... so the plan I had of holding that in a 1/4" 5C collet wouldn't work.... I tried to slide a drill stem inside, thinking I could clamp onto that, and found out that the two setscrews in the top hat had dented the inside of the stem ever so slightly.... just enough that it wouldn't slide over a #3 drill, which was the correct size.... I used a fine 3/16" chainsaw file to carefully remove the bumps from the setscrews until the valve stem would slide easily over the stem of the #3 drill.... I screwed the top hat back into place, located it so the dents in the stem were centered in the setscrew holes, and with the stem of the #3 drill in place, tightened the screws.... I positioned the end of the drill so that it was about 1/8" forward of the cross holes... As I thought, the valve stem collapsed just a bit, gripping the stem of the drill to support it.... That allowed me to tighten the setscrews fairly snug without damaging the valve stem.... I then mounted the front part of the top hat in a 5/16" collet, which I installed in a square 5C collet holder, and turned the stem so that a drill placed through the cross holes was at 90 deg. to the faces of the holder.... It looked like this....

(https://i378.photobucket.com/albums/oo221/rsterne/257%20Condor/Condor%20Stem%20Before%20Small_zpsfsqqng9v.jpg) (https://s378.photobucket.com/user/rsterne/media/257%20Condor/Condor%20Stem%20Before%20Small_zpsfsqqng9v.jpg.html)

I mounted the square collet holder in the milling attachment in my lathe, with the cross holes horizontal.... and with the C/L of the stem on the vertical C/L of the chuck.... I then rotated the milling attachment about the vertical axis until the stem was at a 30 deg. angle to the chuck bore, and mounted a 5/32" end mill in the chuck.... I carefully milled away some material on one side of the stem to change the cross hole from 90 deg. to the stem C/L to just 30 deg.... I went gradually closer to the C/L of the stem until I could feed the mill through into the 0.215" hole in the center of the stem.... At that point, with one side done, it looked like this.... In this end view, looking in the direction of the airflow, you can see how much straighter the flow path is on the right.... In the original setup on the left, you can barely look into the main hole in the stem....

(https://i378.photobucket.com/albums/oo221/rsterne/257%20Condor/Condor%20Stem%20During%20Small_zpsueieuyvn.jpg) (https://s378.photobucket.com/user/rsterne/media/257%20Condor/Condor%20Stem%20During%20Small_zpsueieuyvn.jpg.html)

I then mounted the square collet holder on the other side and repeated the procedure.... ending up with the two cross holes penetrating the sides of the stem at a 30 deg. angle.... That basically straightens the airflow from two 90 deg. bends to just a shallow "S".... hopefully reducing the resistance to flow and the turbulence at that point.... The area of the two 5/32" holes is 7% greater than the area of the main hole through the center of the stem.... Here is what the stem looks like after changing the angle of the holes....

(https://i378.photobucket.com/albums/oo221/rsterne/257%20Condor/Condor%20Stem%20After%20Small_zpsgqpvuyrj.jpg) (https://s378.photobucket.com/user/rsterne/media/257%20Condor/Condor%20Stem%20After%20Small_zpsgqpvuyrj.jpg.html)

There is still quite a bit of metal between the cross holes and the stem to support it.... I'm hoping that is enough strength to survive the rigors of valve operation.... Incidently, once this machining is done, you end up with virtually bore-size passages for a .22 cal Condor.... However, in this .257 cal version the porting is only 84% of the caliber, and 70% of the bore area.... I think I will get an improvement in performance, but don't expect to approach what Doug Nobel gets with his valves.... Fortunately, I have one of those coming to fit a 250 bar (3625 psi) 500 cc aluminum bottle I just happen to have on the shelf.... When I built my .457 Hayabusa, I bought two....  8)

Bob
Title: Re: My .257 Condor Build
Post by: rsterne on January 30, 2020, 07:24:16 PM
This evening I did some testing with the new angled ports in the stem.... First I tried all the Power Wheel settings at 3000 psi, with 2 different bullet weights....

(https://i378.photobucket.com/albums/oo221/rsterne/257%20Condor/257%20Condor%20Anged%20PW_zpsuxnnqpw6.jpg) (https://s378.photobucket.com/user/rsterne/media/257%20Condor/257%20Condor%20Anged%20PW_zpsuxnnqpw6.jpg.html)

I still have the 1 mm Oring in place on the stem, and the top hat was in exactly the same place as before.... The valve acts completely differently.... It is much like a Cothran valve, it either cycles or it doesn't....  :o …. Once the valve is shooting at almost full power, all that increasing the PW setting does is use more air, as you would expect.... With light slugs, there is only 2 turns on the PW to go from 500 fps to over 900, and with the heavy slugs it is even worse, just one turn from 400 fps to over 800.... I shot some strings with the lighter slugs at three different power wheel settings.... I used a 3000 psi fill, as before....

(https://i378.photobucket.com/albums/oo221/rsterne/257%20Condor/257%20Condor%20Anged%20Strings_zpson62zykn.jpg) (https://s378.photobucket.com/user/rsterne/media/257%20Condor/257%20Condor%20Anged%20Strings_zpson62zykn.jpg.html)

At PW = 5 or 6, it was a declining shot string, with only 6 or 7 shots within a 4% ES.... Yes, the power was increased significantly, but tuning for a decent number of shots just didn't happen.... When I tried PW = 4, the first 4 shots were under 750 fps, then it jumped up to 934 when the pressure dropped to about 2900 psi.... The next shot was faster at 943 fps, and then it started to decline again, with only 7 shots within a 4% ES....

Interestingly, the gun wasn't using much air, the pressure at the end of the strings was between 2670-2760 psi.... That meant that the efficiency was astounding (for a Condor), averaging about 1.4 FPE/CI, compared to 1.1 FPE/CI in stock form (with the O-ring)…. Those strings are shown in the dotted lines above, for comparison.... The longer strings with the stock valve stem are because I could shoot down to about 2400-2440 psi before the ES exceeded 4%....

With 82.5 gr. slugs I got over 138 FPE on the first shot (868 fps) but the velocity dropped about 10 fps per shot, and when I backed the power wheel off to try and get a bell-curve then the velocity suddenly dropped by half.... I never got anything but a declinging string of 4-6 shots within a 4% ES....  :-[

So, I am in a bit of a pickle.... I gained a decent amount of power, but now have a gun that can really only be shot tethered.... At 3000 psi it still won't launch the 66 gr. 257420 HPs I have at 950 fps, which is what I get with my Hayabusa…. It has slightly less air (460 cc instead of 500), and a 2" longer barrel (28" instead of 26")…. but the Hayabusa got a nice bell-curve of 10 shots of 935-948-928 (a 2% ES) with the original valve, and with the new balanced valve I have increased the efficiency about 20% with a slight gain in fps as well.... I was hoping that the axial flow valve in the Condor would make up for the 2" shorter barrel.... but with this valve, that is not the case.... It certainly shows the importance of full bore-area porting, which the Hayabusa has and the Condor does not....

I don't know if a change in hammer weight (lighter or heavier, likely lighter) might make a difference in getting a bell-curve or not.... If anyone has any ideas, or has run into a similar problem with a "high-flow" valve in a Condor, please give me a hint....  ::)

Bob
Title: Re: My .257 Condor Build
Post by: rsterne on January 31, 2020, 10:49:18 AM
While I was poking around the internet looking for information on removing the Condor valve, I ran across a YouTube video, and took this screen shot from it....

(https://i378.photobucket.com/albums/oo221/rsterne/257%20Condor/Condor%20Regulated%20Valve_zpsbp542qpz.jpg) (https://s378.photobucket.com/user/rsterne/media/257%20Condor/Condor%20Regulated%20Valve_zpsbp542qpz.jpg.html)

Here is the Video, in interest of full disclosure....
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This is, quite obviously, a regulated valve to fit INSIDE the Condor tank.... The regulator is at the bottom of the tube, and the tube between the regulator and the valve is the plenum....  :o

I have never seen or heard of anything like this before, but it seems a quite brilliant solution for that particular PCP, and in fact could find use in other bottle guns where there is insufficient room for a decent sized plenum.... It does give up some of the reservoir volume, of course.... but the ability to have a decent sized plenum might outweigh that in many instances....

I have a 250 bar 500 cc bottle here that is 3/4" longer than a Condor tank.... That is less length than you would add with an external regulator, which would have NO plenum.... The neck is just over 5/8" ID, and it is 12" long from the inside bottom to the top of the neck.... With care, deducting an inch each for the regulator and valve, that would leave you a maximum of 10" of length for the plenum.... If the wall thickness was 0.065" (ID of 0.495"), the plenum volume would be 32 cc.... That would probably work great for a .22 cal, and could work with a .25 cal by increasing the setpoint about 5-10%, depending on FPE level....

Calculating the external crush pressure of a tube is much more complex than the internal burst pressure, but many I checked seem to be about 40% of the burst pressure.... If we made the assumption that the minimum regulator setpoint pressure was 1600 psi, inside a 3600 psi tank, the greatest pressure differential would be 2000 psi.... Therefore a tube with a decent safety margin to burst with 5000 psi of internal pressure would probably be OK.... Since it is contained inside the bottle, the worst that would happen is that it would flatten, and be impossible to remove from the bottle....

You can get 5/8" OD 2024-T3 tubing in either 0.058" or 0.065" wall, and you can get 4130 CrMoly tubing in those, plus 0.049" wall.... The thinner the wall, the greater the plenum volume, but even going down to 0.049", the volume would only increase to 36 cc.... With the thicker wall, you could use threads to attach the plenum tube to the ends (regulator and valve), as those would provide additional crush support, so the simplest might be to use the 0.065" wall 2024 aluminum.... That should give about a 2:1 safety margin with a 2000 psi differential, so should resist collapse even if the regulator vented.... The same applies to the CrMoly tube at 0.049" wall....

I don't plan on building such a device.... but I thought it was worth posting about it, since I have never seen this idea before....

Bob
Title: Re: My .257 Condor Build
Post by: rsterne on January 31, 2020, 11:34:42 AM
I had one more thing I wanted to try this morning.... I removed the O-ring from behind the Top Hat.... I was wondering if that 1 mm thick, 70D rubber ring was absorbing enough hammer energy to reduce the velocity.... I filled the gun to 3000 psi (for each shot), loaded an 82.5 gr. slug, set the PW to 8 and took a shot across the Chrony….

768 fps instead of 872 with the O-ring.... I repeated at PW = 7 (793), PW = 6 (maximum recorded at 800 fps), PW = 5 (785) and PW = 4 (733 fps)…. I then put the O-ring back on, and checked at PW = 4 and PW = 6, and those shots were 868 & 869 fps....  :o

OK, so that is quite a revelation.... The O-ring appears to be slowing the hammer gently, instead of letting it drive the top hat hard enough to bounce off the valve.... It appears that with the larger, angled ports the breech volume is filling much more rapidly, causing a faster pressure rise.... This drives the top hat back harder/faster, blowing the valve open much more quickly, which is why the valve can't be tuned for a bell-curve now.... If the hammer strike is too light, and barely cracks the valve, it slams shut and you get really low velocity.... Once the valve opens past a certain point, you get that fast pressure rise behind the slug, and the valve gets blown open and you get full power.... almost regardless of the power wheel setting after that....  8)

It appears that the O-ring, by slowing the reversal of the valve stem, increases the dwell and reduces the bouncing closed of the stem, which is limiting the velocity to 800 fps in this case, instead of 870.... This is a rather strange case of an O-ring buffer actually increasing the peak power of a PCP (by nearly 20%)....  :o

Bob

Title: Re: My .257 Condor Build
Post by: Alan on January 31, 2020, 12:23:39 PM
Makes me wonder if the o-ring "size" is specific? As in thicker or thinner?
Title: Re: My .257 Condor Build
Post by: rsterne on January 31, 2020, 02:36:38 PM
It absolutely would be.... I tried a 0.070" thick one before I tried this one (0.039"), and the gun at that time lost a LOT of power.... See previous page, Reply #10....

Bob
Title: Re: My .257 Condor Build
Post by: rsterne on March 06, 2020, 12:56:23 PM
I have spent the last month nursing my dog Molly back to health after her knee surgery.... I am happy to report she is recovering well.... During that time I received a .25 cal Condor valve from Doug Noble (dyotat100) and installed it in a 250 bar (3625 psi) 500 cc bottle that I had lying around.... I did not take it apart, but I did notice that the valve spring is just enough to hold the poppet closed for filling, compared to a very stiff spring in a stock Condor valve.... The strong spring is not required because Doug's valve has a larger stem, but the same size top hat OD (necessary to fit the .25 cal breech)…. so it doesn't have any tendency to "blow open" like a stock valve does.... This means that a heavier hammer and stronger hammer spring are required to allow even the 3000 psi fill of a stock Condor, let alone more pressure for increased performance....

I wanted to find out what Doug's valve could do without upgrading the hammer and spring, and I finally got some time today to do some testing.... After a few shots to find the appropriate fill pressures, I shot two strings, one from a 2400 psi fill with some 59.6 gr. slugs, and the other from 2700 psi with slugs that weighed 82.5 gr.... Here are the results....

(https://i378.photobucket.com/albums/oo221/rsterne/257%20Condor/257%20Condor%20Dougs%20Stock%20Strings_zps3r8xg7xn.jpg) (https://s378.photobucket.com/user/rsterne/media/257%20Condor/257%20Condor%20Dougs%20Stock%20Strings_zps3r8xg7xn.jpg.html)

The FPE levels were significantly greater than stock, and slightly higher than my modified Condor valve with the angled ports in the stem.... but occurred at MUCH lower pressures, showing just how much better Doug's valve breathes.... It does, however, require a lot higher power wheel setting to reach those power levels, even at the reduced pressure.... in fact to get a string starting at 2700 psi required the PW to be maxed out.... There is no doubt that once I fit a heavier hammer and the stronger spring that Doug sent there is much greater FPE lurking inside....

The valve delivers a very nice bell-curve, with 9 shots within a 4% ES for both strings.... The average was 115 FPE with the light slugs and 135 FPE with the heavy ones, but the peak power was delivered at only 2200 psi with the 59.6 gr. and a peak of 140 FPE at just 2400 psi with the 82.5 gr.... My modified Condor valve peaked at 138 FPE at 3000 psi with the 82.5 gr. (and no bell-curve)...  :o

After a few years watching Doug's expertise with the Air Force guns, I didn't see any sense in trying to reinvent the wheel.... I am delighted with the performance of his valve, and have no doubt that with just a hammer and spring change you could tether a .257 Condor at 3000 psi or less and shoot 70-85 gr. slugs as fast as necessary.... Using the extra power available with the change to a 3600 psi bottle will just be the icing on the cake, allowing even heavier slugs to be used, up to the limit of what will work in this 14" twist barrel....

Bob
Title: Re: My .257 Condor Build
Post by: Alan on March 06, 2020, 01:37:01 PM
Summed up?  Innovation!
Title: Re: My .257 Condor Build
Post by: rsterne on March 07, 2020, 03:55:29 PM
I installed Doug's hammer spring today, with the stock hammer.... I know Doug recommends a heavier hammer, but I wanted to see what would happen, and what fill pressure was possible with just the spring change.... The spring is about 10 lbs/in instead of about 4 lbs/in for the stock spring, but it is more than 1/4" shorter....

I started with the power wheel at the minimum setting (1), and filled to 2700 psi and tried the 82.5 gr. bullets I used for yesterday's test.... The velocity jumped to 891 fps, with the second shot slower.... Filled to 2820 psi (what my SCBA tank had in it) I got a 7 shot string, starting at 911, a second shot of 914, and then a decline down to 868 fps at 2400 psi, giving 7 shots averaging 147 FPE.... This compared to the stock spring at PW = 14 (maximum) yesterday, which gave me 9 shots from 2660 psi down to 2120 averaging 135 FPE.... So, Doug's spring at the lowest power wheel setting would allow a higher fill pressure than the stock spring at the highest PW setting.... It was, of course, harder to cock.... I tried various PW settings with the 82.5 gr. bullet at 2820 psi, and the velocity peaked at 931 fps (159 FPE) at PW=4, and then declined after that.... I assume the hammer was bouncing off the Delrin spacer under the top hat on Doug's valve when I cranked the PW over 4 at 2800 psi....

I then got out some 92.6 gr. Lyman 257464 RN bullets, to see what would happen with a heavier weight and more pressure (my other SCBA tank was full)…. The gun was even harder to cock at PW=6 (although certainly OK), so I started there at 3100 psi, and got a first shot of 922 fps (174 FPE), and then 910 for the second shot.... This means that even with the stock hammer, Doug's spring will allow a higher fill than that.... I tried PW=4 at 3000 psi (910 fps, followed by 898), and then dialed the PW back to 2, refilled to 3000 psi, and shot an 8 shot string which started at 858 fps, peaked at 880, and then fell back to 845 fps (a 4% ES), leaving 2420 psi in the 500 cc bottle.... That 8 shots averaged 154 FPE from a 3000 psi fill, with an efficiency of 1.01 FPE/CI....

So, Doug's spring, installed with the stock hammer, can easily allow a 3000 psi fill at near minimum spring preload.... In fact, with a lighter bullet, like the 257420, even with the PW set to minimum you may not be able fill to 3000 if you want a bell-curve, although I didn't try that.... If you want to shoot 70-75 gr. bullets, you may not need to change the hammer, and just use Doug's .25 cal valve and his hammer spring.... It may even be possible to shoot 80-90 gr. bullets with a 3600 psi fill with the stock hammer, but you will have to crank up the power wheel, and the gun will be getting hard to cock.... In that case, a heavier hammer will allow you to reduce the PW setting, which is the route Doug recommends.... Again, I never tried over 3100 psi today....

I'm very pleased with Doug's valve and hammer spring.... I have already seen 170 FPE at 3000 psi, even without a hammer change....

Bob
Title: Re: My .257 Condor Build
Post by: rsterne on March 08, 2020, 01:54:08 PM
Today I increased the pressure to 3600 psi to see how Doug's Valve and Spring worked with the stock hammer.... I did the testing with the 92.6 gr. Lyman 257464 RN bullet, which is about as heavy as you can go with the 14" twist barrel.... First I tried numerous power wheel settings, with the following results....

(https://i378.photobucket.com/albums/oo221/rsterne/257%20Condor/257%20Condor%20Dougs%20PW_zps2u1svt8y.jpg) (https://s378.photobucket.com/user/rsterne/media/257%20Condor/257%20Condor%20Dougs%20PW_zps2u1svt8y.jpg.html)

As you can see, the results show a similar pattern to a stock Condor.... Once you get to maximum power, increasing the PW setting causes the top hat to bounce off the Delrin stop on the valve stem, and that bounce reduces the dwell and therefore the FPE.... The velocity peaked at PW = 10 at 969 fps (193 FPE)…. My "lofty goal" calculation for 3600 psi in a .257 with a 26" barrel is 202 FPE, so that gets to within 4.5%, which is pretty good.... I backed off the PW to 8, and started a string from 3600 psi.... It was 950, 958, 946, 938 and 920 before it dropped more than 4% from the peak, so I reduced the PW setting to 7 and shot two strings, both from a 3600 psi fill.... They are shown below in red.... all strings with a 92.6 gr. bullet....

(https://i378.photobucket.com/albums/oo221/rsterne/257%20Condor/257%20Condor%20Dougs%20Strings_zpsxkgblsxn.jpg) (https://s378.photobucket.com/user/rsterne/media/257%20Condor/257%20Condor%20Dougs%20Strings_zpsxkgblsxn.jpg.html)

You will note that the velocity of the first shot varied, and they were either side of the 925 fps I got when I tested the velocity a PW = 7 above.... This means that I need just a whisker more hammer spring preload ( PW = 7.3? ) for a 3600 psi fill, to avoid those large swings on the first shot.... I ran out of bullets, but it is pretty obvious from the 2 strings that with a little fine tuning I can get 7 solid shots within an ES of less than 3%.... The average over the 15 shots of those two strings was 938 fpe (181 FPE) at 1.15 FPE/CI.... I am very pleased with that efficiency at this power level.... especially considering the Condor does not have a stellar reputation for efficiency....

Although Doug told me that I should make a heavier hammer, I seriously doubt that would actually give an increase in power, because the hammer is already bouncing off the valve, and more than likely it would drop the efficiency.... It certainly would reduce the cocking effort by allowing me to use a lower PW setting, but on the other hand it would likely mean that I would not be able to reduce the PW far enough to shoot a lighter bullet, such as the 257420, without having the velocity pushing over 1000 fps (or having a declining shot string)…. If you look at the black line on the chart above, you can see what happens with an 82.5 gr. bullet with a 3000 psi fill at PW = 2.... That setting would push the 257420 into the mid 900s, so Doug's spring, with the stock hammer, gives me a nice adjustment range between the minimum PW setting and PW = 8 to handle all weights of bullets and pressures up to 3600 psi.... I think I'll stay right where I am for now, as I don't think a heavy hammer would buy me any more FPE with this 26" barrel....

Bob
Title: Re: My .257 Condor Build
Post by: Alan on March 08, 2020, 02:51:55 PM
This raises a question. The most important adjustment on several FX airguns, is the hammer stop. I wonder if you have ever tried to duplicate the hammer stop adjustment on any of your home-brew airguns??

Title: Re: My .257 Condor Build
Post by: rsterne on March 08, 2020, 03:56:15 PM
It can only be used successfully on a regulated PCP.... some unregulated Korean guns use it, and it is one of the primary reasons for the "Korean Cliff" tune, where every shot is lower velocity as the pressure drops.... Having said that, the bstaley O-ring buffer mod, which is intended for unregulated PCPs, "sort of" works that way, but it actually works as a progressive valve spring instead of a rigid stop....

I have never used it on a regulated gun either, although I can see it's usefulness.... It is another way to get a precise mount of lift, while allowing somewhat more hammer strike than necessary to produce that lift.... This tends to "square off" the lift to dwell curve, which is a good thing.... as in theory at least it should improve the efficiency.... However, it means that to change the tune to any significant degree, you need to adjust 2 or 3 (or 4) things, instead of just one....

If you have a hammer stop, when you increase the hammer spring preload past the point where the hammer is hitting the stop, the hammer bounces off the stop, actually decreasing the dwell.... You can see that in the upper graph in my last post on the Condor.... Without the stop, there may actually be a bit more velocity available, but you can't find it without moving the stop.... Without a stop in place, all you need to do is increase the preload until the velocity plateaus, and you then know FOR SURE what the maximum is.... Since I always tune my PCPs about 3-5% below that velocity, then the hammer would never hit the stop, if it were set with enough clearance to allow maximum FPE....

If I saw PCPs with a hammer stop greatly exceeding the efficiency I can get without one, then I might try it.... The one benefit there may be, is that you should be able to tune for a "downslope" tune without getting a large shot-to-shot variation, and that "hump" in velocity once the pressure drops below the setpoint.... You give the gun a bit more preload than necessary, and then adjust the stop to drop the velocity slightly, making the gun operate more consistently if tuned on the downslope.... I never tune my guns on the downslope (instead I reduce the setpoint pressure)…. so I don't need that feature....

Bob
Title: Re: My .257 Condor Build
Post by: rsterne on March 11, 2020, 12:00:18 PM
Today I double-checked the power wheel settting to confirm that maximum FPE occurred at PW = 10 wth a 3600 psi fill.... I tested it with bullet weights of 62 gr. and 105 gr., and that was the case with both weights.... I used that setting and tethered the gun at 3600 psi to find the velocity of every possible pellet and bullet I had in .257 cal.... The pellets I used were .25 cal, but the skirts were 0.257" or so, and I was confident that at 3600 psi they would blow out to seal just fine, and that proved correct.... Here is a chart showing the maximum velocity and FPE I am getting at 3600 psi with my .257 Condor with Doug's valve and spring....

(https://i378.photobucket.com/albums/oo221/rsterne/257%20Condor/257%20Condor%20Dougs%20Vel%20FPEjpg_zps4xp1hlsn.jpg) (https://s378.photobucket.com/user/rsterne/media/257%20Condor/257%20Condor%20Dougs%20Vel%20FPEjpg_zps4xp1hlsn.jpg.html)

Those below 42 gr. were pellets.... Note that this gun will push the 25.4 gr. JSB Kings at 1420 fps.... All the pellets were supersonic, I just did this out of curiousity…. Also note that the heaviest three bullets were too long to stabilize, and were tumbling by the time they hit the backstop, which was only 6 yards behind the Chrony….  :o …. This leaves a usable range of bullet weights from 58-93 gr., which produce between 160-190 FPE, with the heavier bullets having more energy, as expected.... The NOE 73.9 gr. boattail version of the 257420 shot at 1041 fps (and the HP at 1051), which may or may not be usable.... but if I have to slow it down it is a simple matter to dial back the power wheel, and/or drop the pressure a bit.... The shortened HP version, at 58.3 gr., was just supersonic (1124 fps) at PW = 10 and 3600 psi.... but if I want to use those I can easily drop the pressure down to 3000 psi and back off the power wheel accordingly....  The gun is capable of hitting the high 900s with an 85 gr. bullet, and could be backed off a bit to save air....

One other thing I determined is that PW = 8 looks like the perfect setting for tethering 80-90 gr. bullets at 3500-3600 psi, the gun shoots a 4-shot string starting from 3600 within a 2% ES, getting over 1.0 FPE/CI at that setting.... At 3500 psi and PW = 8 it is right at the top of the bell-curve.... All in all, it looks like this gun will make a very capable varmint and target rifle.... providing the accuracy matches the power....

Bob
Title: Re: My .257 Condor Build
Post by: rsterne on March 12, 2020, 04:31:26 PM
I have always been curious about how the diameter you size a bullet to effects the velocity.... so today I used the Condor and 3 different bullet weights, and tried every size from 0.251" to 0.259", shot 3 shots with each bullet and determined the average velocity.... I then plotted that vs. the diameter....

(https://i378.photobucket.com/albums/oo221/rsterne/257%20Condor/257%20Condor%20Vel%20vs%20Diam_zpsuxu0ypm8.jpg) (https://s378.photobucket.com/user/rsterne/media/257%20Condor/257%20Condor%20Vel%20vs%20Diam_zpsuxu0ypm8.jpg.html)

The highest velocity was at 0.257", which corresponds to the groove diameter of the barrel.... There was essentially no difference between 0.256" and 0.258", both being within 1-2 fps.... With the heaviest bullet, even sizing to 0.255" or 0.259" really didn't make more than 2-3 fps difference in the velocity, but with the lightest bullet both were down about 5-6 fps (still pretty much lost in the ES)…. Where a bit of blowby became obvious in reducing the velocity was at 0.254" and smaller.... The velocity loss was more pronounced with the lighter bullet, which likely has something to do with the relationship between mass and bore friction compared to the heavier one....

I have seen similar results to this before, but this is the first time I actually set out to find out how much blowby affected velocity.... and the answer is not much.... Basically all you need to worry about when trying different sizing is the accuracy.... Anywhere from 0.002" smaller to 0.002" larger than the bore makes virtually no difference to the velocity.... Even 0.003" under only looses 1-2%....

Bob