Author Topic: Tale of THREE BRods - .25, .30 and .357 Completed, Page 12 !  (Read 22192 times)

rsterne

  • Member 2000+fps Club
  • Global Moderator
  • Hero Member
  • *****
  • Posts: 1825
  • Mozey-On-Inn and see what Coalmont has to offer!
    • Mozey-On-Inn
Re: Tale of Two BRods
« Reply #45 on: January 18, 2017, 06:40:22 PM »
Seems like most of the 51 gr. bullet shooters in .25 cal peak out at around 115 FPE, depending on the actual pressure used.... I got the same with my .25 cal Disco at 2900 psi, and did 112 in this build at the same pressure with the Cothran valve.... The only way to get a bit more FPE without a longer barrel or more pressure appears to be a heavier bullet.... My Disco pushed to 121 FPE with a 58.8 gr.... By the time you tune it to a usable shot string, your results are typical as well.... I got 5 shots inside 5% at 96 FPE average with my Disco using the stock 135 cc tube....

Bob
  • Coalmont, BC

Rdsail

  • Jr. Member
  • **
  • Posts: 69
Re: Tale of Two BRods
« Reply #46 on: January 18, 2017, 07:09:18 PM »
Interesting about the similar results.  In retrospect I should have gone up to .257 or even .30 but it was an experiment.

My biggest problem with the build was taming the hammer. That is were the cothran valve should help by reducing the hammer energy.

I'll stay tune on your build! By far you give the best detail and scientific notations.
  • Richmond, VA

rsterne

  • Member 2000+fps Club
  • Global Moderator
  • Hero Member
  • *****
  • Posts: 1825
  • Mozey-On-Inn and see what Coalmont has to offer!
    • Mozey-On-Inn
Re: Tale of Two BRods
« Reply #47 on: January 19, 2017, 06:16:27 PM »
Today I hooked the gun up to a regulated 500 cc tank so that I could take pressure readings on that tank as I shot some short strings to check the efficiency.... I started with the tube with my valve in it, with the MDS hammer.... I adjusted the SSG to zero gap, shot one 8-shot magazine using the 34.2 gr. JSB Heavies, and recorded the average velocity and the pressure drop.... I then turned the SSG gap adjuster out one turn, repeated, and then did the same thing at 2 turns of gap.... I then changed to the 25.4 gr. Kings, shot another magazine with those, and continued, using the lighter pellets, for 2 more turns.... I then replaced the MDS hammer with the steel one, and starting from 4 turns out (where the starting velocity was the same, 945 fps with the 34 gr.), kept going, repeating the above with the steel hammer.... Then I calculated the FPE/CI based on each pressure drop and the total FPE of each string.... and plotted all the data as below....



The efficiency is very similar with the steel and MDS hammers.... The gauge I was using to monitor the 500 cc tank had 100 psi increments, so all pressure reading are only within about 10 psi at best, but I feel that the trends above are valid, even if the exact values are not perfect because of the 8 shot strings, which resulted in pressure drops from 120 to 240 psi over the 8 shots.... More accurate numbers would require longer strings..... I also averaged the FPE/CI values at the end of the 34 gr. string and the beginning of the 25 gr. string, because they were pretty close (the heavy pellets were a bit better).... The important numbers, of course, are what the efficiency is at the highest velocities tested (945 fps with the 34 gr. JSBs).... 8 shots used 240 psi with the MDS hammer and 230 psi with the steel hammer, so you can call it a wash (that's as close as I can read the gauge).... That works out to 1.08 - 1.12 FPE/CI at 68 FPE.... and that means I can get about 38 shots per tank from 3000 psi down to the 1850 psi setpoint I was using....

I then swapped tubes, fitting the one with the Cothran valve and the steel hammer.... I had to back the SSG wayyyyyyyyy out to find the cliff, because the last time I used that valve, I was getting a vacuum behind the hammer, which I have since cured.... With the steel hammer, the cliff occurred at just over 8 turns of gap on the SSG, and with the MDS hammer at just over 6 turns.... I followed the same procedure as above, but only using the 34 gr. pellets, with the following results....



I did not record the pressure drop, or calculate the efficiency, after the velocity fell off the cliff, as there was no point, you would never operate the gun there.... I did find that the difference between the valve opening properly and intermittently (fast and slow shots mixed) was only one flat (1/6 turn) with the MDS hammer, and then 1 more flat put the velocity in the basement.... With the steel hammer, just 1 flat went from full power over the cliff.... You can see that as you approach the cliff from the high-velocity end, the efficiency rapidly increases, particularly over the last turn.... A couple of turns too much on the SG, and the efficiency is a dreadful 0.45 FPE/CI, with the gun using about 80 psi per shot.... The best efficiency occurs just above the cliff.... For 8 shots, the best I could do was 290 psi drop with the MDS hammer and 310 psi with the steel hammer.... Again, those are close enough that there may not be any real advantage to either hammer.... It works out to 0.88-0.94 FPE/CI, which is significantly less than I got with my modded valve.... That works out to 30-31 shots per tank, compared to 38 with my valve.... Note, however, part of that will very likely be the fact that the velocity with the Cothran valve was greater.... 970 fps (72 FPE) compared to 945 fps (68 FPE) for my valve.... If they were tuned to the same velocity, the difference in efficiency may well disappear.... However, that would require running the Cothran valve at a lower pressure than my valve.... likely around 1700 psi instead of 1850, to drop it down to 68 FPE.... That would increase the headroom (pressure difference between fill pressure and setpoint) from 1150 psi to 1300, which in itself would raise the shot count to 34-35 shots per fill.... If there was any increase in efficiency, it wouldn't take much to get back to the 38 shots from my valve.... In all fairness, I would have to say that the efficiency of the two valves is VERY similar.... so much so that a tiny tweak one way or the other could make either valve come out on top....

There is another variable that will likely effect the ultimate FPE/CI numbers.... For all versions except my valve with the MDS hammer, there was a lot of gap in the SSG.... A better solution would be to fit a weaker hammer spring to reduce the gap.... Small gaps generally make for narrower ES values.... I would use the MDS hammer with the Cothran valve, in addition to a lighter spring.... the valve takes that little to open it.... One other thing that could be tried with the Cothran valve would be to artificially limit the lift of the valve by using a bumper on the back of the valve, and an adjustable, recessed hammer face, like Lloyd was experimenting with.... It seems to me that the Cothran valve, as is, simply releases too much air to be efficient in .25 cal. with pellets.... To get the velocity low enough you have to drop the setpoint pressure down a long ways.... I have a gut feeling that using a higher pressure, say 2000 psi instead of 1700, and then limiting how far the valve can open with a stop, may increase the FPE/CI.... I hope Lloyd continues his experiments with that at some point.... Using my valve, I think bumping the pressure up a bit, back to 1900 psi, or maybe even 2000.... and then operating the valve further down on the knee.... will improve the efficiency of that as well.... and may well buy me a few shots below the setpoint, giving me a similar shot count to what I have now, at my 70 FPE goal....

Bob
  • Coalmont, BC

PakProtector

  • Newbie
  • *
  • Posts: 19
Re: Tale of Two BRods
« Reply #48 on: January 22, 2017, 05:05:58 AM »
As much as I hated to move things around, when I built the 3/8" transfer port( OD's )Marauders, I moved the port center line forward .015" just to get a bit more wiggle room between the front side of the o-ring groove, and the back side of the transfer port spot face.
cheers,
Douglas

rsterne

  • Member 2000+fps Club
  • Global Moderator
  • Hero Member
  • *****
  • Posts: 1825
  • Mozey-On-Inn and see what Coalmont has to offer!
    • Mozey-On-Inn
Re: Tale of Two BRods
« Reply #49 on: January 22, 2017, 08:12:39 AM »
Yes, it's extremely tight there.... can be done, but the position of the O-ring groove becomes critical.... If you are working from scratch (valve, tube, and receiver) it's easy to move it ahead.... but one of my valves (the Cothran) was already built, so I had no choice.... unless I wanted to make an offset transfer port like Lloyd did on his first Cothran valve build....

Bob
« Last Edit: January 22, 2017, 08:16:30 AM by rsterne »
  • Coalmont, BC

rsterne

  • Member 2000+fps Club
  • Global Moderator
  • Hero Member
  • *****
  • Posts: 1825
  • Mozey-On-Inn and see what Coalmont has to offer!
    • Mozey-On-Inn
Re: Tale of Two BRods
« Reply #50 on: January 23, 2017, 06:42:20 PM »
Today I moved back the port in the .30 cal barrel that I had to line up with the port in the MRod receiver.... I was able to get everything to line up without having to cut another O-ring groove, shorten the barrel and machine a new chamber, so that saved me a LOT of time.... The new barrel port is 0.22" wide by 0.28" long, so it is the same area as the 1/4" transfer port.... and the new spot face around the port is 3/8" to match the receiver, valve, and the OD of the transfer port.... The barrel was already machined for a Hatsan Air Stripper, so with just the change in port location it was ready to fit with a Carbon Fibre sleeve to stiffen it.... The barrel is a TJ's with a 1/2" OD and I had some high modulus CF tubing of that ID that was 16mm OD, the same as the .25 cal LW barrel I used, which I had already machined the receivers to fit.... By insetting the CF tube into the receiver, there is no weak area at the front of the receiver for the barrel to bend, and the high-modulus CF the tube is made of is 50% stiffer than normal CF, which puts it at least as stiff as a steel barrel of the same diameter.... Instead of being able to flex the barrel with your hands, once sleeved, it's like a rock....

I sanded the barrel OD and the ID of the tube with 220 grit to give a bit of "tooth" for the adhesive.... I used Loctite 638 (green) which is high-strength, high-viscosity, intended as a retaining compound for slip-fitted parts with a gap of 0.010" or less.... It has a shear strength of 4500 psi once cured, stronger than most epoxies, has a working time of 4 minutes (or less) and a curing time of 24 hours.... Once cured, I will be giving it a post cure in my wife's oven at 175*F for 3 hours to assure full strength.... You MUST assure that the parts slide together nice and easy, if they stick and you have to force them, you may run out of time, because the tighter the fit, the quicker 638 sets to the point you can't move it.... I used two nuts on the threaded muzzle portion, and an O-ring, as a stop for the sleeve, to insure proper placement.... but a few wraps of masking tape will also work just fine.... Make sure you have the CF tube exactly the length you want, and know where you want to position it, before you apply any glue.... Make sure you plug the ends of the barrel and any ports, with wadded up paper towel, etc., to keep the glue out.... Here is what it looks like with the CF tube glued in place....



The portion of CF tube in the receiver is about the same length as the steel part where the port is.... I had to drill out the cone of the Hatsan Stripper to 11/32" to have ample clearance for the .30 cal bullets.... This one started out drilled for a .177 cal.... The other job I did today was to change the cocking and lockup pin in the bolt to 10-32 for safety with the extra loads from the .30 cal.... At the same pressure, the force pushing the bolt back is 44% greater than on a .25 cal, and going up from 8-32 to 10-32 increases the shear area by 45%.... In the .30 cal, at 3000 psi, there is 212 lbs. force on the bolt, and although the stock 8-32 should be able to handle that, if you ever fired the gun without having the bolt handle down, that force would accelerate the bolt backwards, with only that 8-32 bolt to stop it before it hits your face.... We have never heard of Crosman having a problem, but the MRod only goes to .25 cal from the factory.... Using a 10-32 gives me the same safety margin with my .30 cal.... Here is a photo of the receiver showing the new pin and the wider slots for it....



The MRod bolt is case hardened, so the first thing I had to do was heat it red hot around the threaded hole for the pin, and from there forward.... By letting it cool slowly, it anneals the steel, making it much easier to machine.... although there is still a bit of a tough skin.... I carefully lined up the 8-32 hole with my lathe chuck, using a long screw in the chuck, and clamped down the bolt.... I then removed the screw from the chuck, and drilled the hole out and tapped it 10-32.... One tap I had was a bit older, and I thought I might break it, as the metal was still hard to thread.... but my other tap, a nice new one, cut the threads no problem.... I then used a 1/2" long 10-32 SHCS, and loctited on a hex nut, tightened hard against the head of the screw.... I turned the nut, and the head of the screw down to 1/4" OD, and shortened the head about 0.025" so that it was just below flush in the receiver when installed.... I had to mill out the cocking slot in the receiver wider as well, along with the cross-slot on the left side (I will only be installing the handle on the right) so that the new, larger locking pin would fit....

The last thing I did today was to grind off the probe and the nose of the bolt, leaving a 1/4" stub about 3/4" long.... I will make a new .30 cal nose with probe that will be slid over the stub and loctited in place to complete the conversion of the receiver to .30 cal....

Bob

  • Coalmont, BC

oldpro

  • Guest
Re: Tale of Two BRods
« Reply #51 on: January 23, 2017, 07:23:33 PM »
 Super close now!!!! Keep rocking your almost there.

rsterne

  • Member 2000+fps Club
  • Global Moderator
  • Hero Member
  • *****
  • Posts: 1825
  • Mozey-On-Inn and see what Coalmont has to offer!
    • Mozey-On-Inn
Re: Tale of Two BRods
« Reply #52 on: January 24, 2017, 06:32:20 PM »
I finished up the .30 cal Bolt conversion today by increasing the diameter of the forward section.... Now you can clearly see the larger cocking / locking lug....



I ground off the probe, leaving a straight 1/4" diameter shank 3/4" long.... and then made a sleeve to slide over the 1/4" diameter shank from a piece of 5/16" drill rod.... I turned the front part down to form a probe that is 7/64" in diameter, which just fits inside the base of the JSB 45 gr. and 50 gr. pellets.... The flow area remaining around the probe is the equivalent of a 0.280" port, and the ports in the barrel and Cothran valve are only 1/4", so it should not restrict the flow.... I turned down (filed and sanded, actually, in the lathe) the OD so that it would just slide into the chamber of my .30 cal barrel, checked the alignment, and glued it onto the stub of the bolt with Loctite 638 (green) and allowed it to set....

The shoulder on the bolt nose is located so that the outside edge of it is flush with the back of the barrel port.... and the probe will just push the skirt of the JSB pellets past the front of the barrel port.... When you are making a longer, thinner probe, you are limited in the overall length by how far it will withdraw to clear the back of the magazine.... With the hammer and cocking pin in place, I temporarily install the receiver, with the bolt in place, and pull it back as far as the hammer will allow it to go.... and then mark and cut the probe off flush with the back of the magazine notch.... That will still be a bit long, once you install the trigger, the hammer when cocked will sit slightly forward of that point, and you may have to shorten it a bit more.... The problem is, that if you do, it may not push pellets past the longer barrel port, so you may be stuck with having to hold the bolt back to remove the magazine.... On my .25 cal, I am just able to remove the mag. with the gun cocked, I won't know on this one until the final assembly.... Worst case, I don't mind holding the bolt back to remove the mag. if that is the only way to get the pellet past the barrel port....  Like with many conversions, there is often just enough (or not quite enough) room for things like this....

Bob
  • Coalmont, BC

bnowlin

  • Hero Member
  • *****
  • Posts: 515
  • Just a Plain Ole Country Boy
Re: Tale of Two BRods
« Reply #53 on: January 24, 2017, 07:49:15 PM »
Gosh Bob, great work. You never cease to amaze me.
Bob the other one

Motorhead

  • Sr. Member
  • ****
  • Posts: 438
  • Older & Slower ... Field Target Shooter
Re: Tale of Two BRods
« Reply #54 on: January 24, 2017, 09:36:05 PM »
Bob,
Am I reading that right ? ..... Breech to Probe seal is that of a Close fit between the two having no mechanical seal present ?
  • Old Hangtown ... California
"Home of Motorheads Air Gun Tuning Service"
    ***** Airguns need love too !! *****

rsterne

  • Member 2000+fps Club
  • Global Moderator
  • Hero Member
  • *****
  • Posts: 1825
  • Mozey-On-Inn and see what Coalmont has to offer!
    • Mozey-On-Inn
Re: Tale of Two BRods
« Reply #55 on: January 25, 2017, 08:10:08 AM »
Don't know where you got that?.... The barrel has a conventional O-ring inside it, per usual MRod practice.... same as I did on the .25cal.... The barrel had already been machined for an earlier project and I guess I just forgot to mention the groove....  :-[

Bob
  • Coalmont, BC

Motorhead

  • Sr. Member
  • ****
  • Posts: 438
  • Older & Slower ... Field Target Shooter
Re: Tale of Two BRods
« Reply #56 on: January 25, 2017, 08:15:25 AM »
Lol ... or just me reading between lines  ???
  • Old Hangtown ... California
"Home of Motorheads Air Gun Tuning Service"
    ***** Airguns need love too !! *****

rsterne

  • Member 2000+fps Club
  • Global Moderator
  • Hero Member
  • *****
  • Posts: 1825
  • Mozey-On-Inn and see what Coalmont has to offer!
    • Mozey-On-Inn
Re: Tale of Two BRods
« Reply #57 on: January 25, 2017, 06:08:02 PM »
I assembled the .30 cal version today, using the tube with the Cothran valve and the MDS hammer, and did some test shots tethered at 1900 psi and also at 2900 psi.... After about 20 shots at 2900 I started to get some blowby, and within 3 shots, it was severe, with a 200 fps velocity drop.... I pulled the gun apart to find this....



Yep, that's whats left of the Teflon Transfer Port.... Looking at the damage, I think the bottom edge which was thinned out to clear the O-ring in the Cothran valve failed, which let HPA out between the valve and the OD of the port, which then collapsed the port.... Interesting failure, anyway, and the first time I have had a Teflon TP do anything strange.... If there was no O-ring in the Cothran valve, and the TP tube was full thickness to the bottom, I think it would have been fine, as I have used them at much higher FPE and pressure levels.... Anyway, I'll have to come up with a solution, maybe just use PEEK?....

I was quite pleased with the numbers I achieved.... I tested both the 45 gr. and 50 gr. JSB pellets at 1900 psi, and then tested some NOE 67 gr. BBTs and also some Accurate 71 gr. BBTs at both pressures.... Here are the results, with the SSG gap adjusted so that the valve is operating just above the cliff.... NOTE - I EDITED THE TABLE BELOW AFTER MAKING A BRASS TRANSFER PORT - I GUESS THE TEFLON ONE WAS FAILING, BECAUSE I GAINED 50 FPS ON THE 2900 PSI SHOTS....




These represent some pretty impressive power at those pressures.... I now have to make the same tests on the other tube assembly, with the steel hammer and my modded valve.... I got my new NOE .253 cal 51 gr. BBT mould today, so once I get some cast up I will be able to see what these guns will do with those at 2900 psi.... I also need to try the 41 gr. BBTs as well.... so lots of testing ahead yet....

Bob
« Last Edit: January 25, 2017, 08:00:58 PM by rsterne »
  • Coalmont, BC

caniborrowsomeammo

  • Administrator
  • Hero Member
  • *****
  • Posts: 766
Re: Tale of Two BRods
« Reply #58 on: January 25, 2017, 06:22:36 PM »
Bob, I turn my transfer ports out of 3/8 acetal stock. Isn't it (acetal) more rigid than teflon?
Buncha BB guns that I don't get to shoot as much as I like. Grinding out a living on the farm.

Dairyboy

  • Hero Member
  • *****
  • Posts: 887
Re: Tale of Two BRods
« Reply #59 on: January 25, 2017, 06:28:14 PM »
Bob, I turn my transfer ports out of 3/8 acetal stock. Isn't it (acetal) more rigid than teflon?
Still have yours in my .25 Brod John works great  ;)
  • Outlook, WA
John 3:16

.22 SPA M10
.22 JSAR Raptor Mini (on order)
.22 AAA EVOL Tac (on order)