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Topics - rsterne

Pages: [1] 2 3 ... 8
1
There have been many discussions about downrange ballistics, and since Helium has come on the scene, we now can play with velocities that were unheard of not that long ago.... I have done up some charts to compare what happens with a 100 gr. bullet with a BC of 0.200, compared to a 200 gr. bullet with a BC of 0.400.... The caliber is irrelevant, as long as it is constant, but because of the weights chosen, it might help if you think about a .308 cal powered by Helium for the higher velocities (or by air for the lower ones).... I used the G1 Ballistics Model, so think of these as flat-based Spitzers.... The BC is the important number, along with the muzzle velocity....

If you shoot a 100 gr. bullet at 1500 fps, that is 500 FPE, and if you double the bullet weight, about the best you might expect is about 1100 fps, which is 537 FPE.... Conversely, if you tune the gun to shoot a 200 gr. at 500 fps, you might hit around 700 fps with a 100 gr.... That is why the charts for the light bullet use a range of MVs from 700-1500 fps.... while the charts for the heavy bullet show a range of 500-1100 fps.... I am simply trying to get a valid comparison between a light bullet driven fast and a heavy bullet driven at just slightly higher FPE, just as you might achieve in reality.... Anyways, here are the charts for what is happening at 200 yards downrange in a 10 mph crosswind....



Note that even starting at 1500 fps, the bullet is subsonic at 200 yards (it has lost 450 fps).... and has lost just over half it's energy.... However, if you start it at only 1000 fps, it has only lost 150 fps, and retains over 70% of it's energy....



In the second chart, the bullet drops about twice as far starting at 1000 fps than it does at 1500 fps (80" instead of 40").... but it only has about 10" of drift instead of 15"....



By comparison, the heavy bullet (with it's superior BC) retains 79% of its energy starting at 1100 fps, and 86% starting at 900.... It has only lost about 120 fps when starting at 1100 fps, and only about 80 fps if launched at 900....



In the last chart, you will note that for a given muzzle velocity, the heavier bullet, with its superior BC, has a flatter trajectory compared to the lighter bullet in the second chart.... In fact, if you compare the heavy bullet at 950 fps to the light one at 1000 fps, they have the same drop.... and the heavy bullet has 5" of drift, compared to 10" for the light one....

My personal preference is to use whatever bullet your gun will shoot in the 900-1000 fps range.... I know that some prefer a lighter bullet moving faster, and that's fine too.... All I am trying to convey here is what happens if you take that into the Supersonic range, where the drag dominates bullet performance.... Yes, you can get a flatter trajectory, as after all, velocity is the dominant factor in determining that.... much moreso than the BC.... There are compromises to be made, however.... and hopefully the above will give you something to ponder to determine what those are....

Bob


2
This is a project I have been developing on the GTA for a while now.... Here is a rough drawing of the concept, which is to mount two empty 88 gr. CO2 cylinders under a QB78 main tube, reversed back towards the trigger group.... This will provide a nice wide base to the forearm for shooting off bags on a bench.... A pair of cylinders weigh 1 lb. 9 oz. and they each have an internal volume of 121 cc....



Well I finally got some shop time today, and I decided to work on the tank block to mount two 88gr. CO2 cylinders reversed under a QB78 main tube.... Using the previously posted drawing as a basis, I produced the following....



The upper hole is tapped 1/4" NPT for a threaded stub that will mount the block to the front of a QB78 main tube, retained using the original threaded CO2 cap.... You can see the two air passages crossing in the bottom of the hole.... They were drilled with a #21 drill (0.159"), which is the tap drill size for a 10-32 screw.... The total area of the two passages is equal to a 0.225" hole, so a similar size hole will be drilled through the 1/4" NPT stub to feed air into the QB78 tube.... Since that is larger than anything that would be used for the exhaust port, that means the entire 290+ cc's will be available to the valve....



There is an 11/32" hole drilled through in line with each 88 gr. cylinder, and that is tapped from the front 1/8" NPT for a gauge and a male Foster fill fitting.... The air passages will be plugged with low-profile 10-32 SHCSs, sealed with a #008 O-ring in the 5/16" recesses milled in the lower corners of the block.... At the moment the holes for the 88 gr. cylinders are only milled 1/2" diameter and 3/4" deep.... I will be centering each hole in a 4-jaw chuck in my lathe to machine the O-ring groove, and drill and tap the portion outboard of the O-ring to 16 mm x 1.5 mm to accept the cylinders....

The block weighs 5 oz., so the completed weight with the stub and the two cylinders should be just under 2 lbs.... The stock will have to be shortened a bit, so the total weight increase will be less than that.... I plan an operating pressure of 1800 psi maximum, in keeping with what CO2 can reach at 120*F, and because that is the MSWP of CO2 cylinders....

Bob

3
Airgun Classifieds / Wanted QB78 Tube and Front Cap - Got It !
« on: November 19, 2017, 11:42:25 AM »
For an experimental project, I need a QB78 tube and the front cap that threads over it and retains the CO2 plug.... If you have one you wish to donate, please PM me....

Looks like I have one coming.... Thanks for looking....


Bob

4
Airgun Ammo / .225 cal 35 gr. RF & HP at NOE
« on: November 19, 2017, 08:54:52 AM »
New Group Buy at NOE for a 35 gr. GC bullet in .225 cal, and also a smooth sided 37 gr. version.... both with a Lyman HP pin available in one cavity.... http://noebulletmolds.com/smf/index.php/topic,2298.0.html

Bob


5
Airgun Ammo / Need BBT HP Moulds?
« on: November 13, 2017, 08:32:34 AM »
The NOE Store has sold out of the HP versions of the .22 and .25 cal Bob's Boattail Moulds with a Lyman HP pin in one cavity.... If you are interested in seeing an inventory run, please post in this thread....  http://noebulletmolds.com/smf/index.php/topic,2316.0.html

Please state the size(s) that you are interested in....

Bob

6
OFF TOPIC / We Will Remember Them
« on: November 11, 2017, 08:09:30 AM »
High Flight

John Gillespie Magee, Jr

Oh! I have slipped the surly bonds of earth,
 And danced the skies on laughter-silvered wings;
 Sunward I've climbed, and joined the tumbling mirth
 Of sun-split clouds, --and done a hundred things
 You have not dreamed of --Wheeled and soared and swung
 High in the sunlit silence. Hov'ring there
 I've chased the shouting wind along, and flung
 My eager craft through footless halls of air...
 Up, up the long, delirious, burning blue
 I've topped the wind-swept heights with easy grace
 Where never lark or even eagle flew --
And, while with silent lifting mind I've trod
 The high untrespassed sanctity of space,
 Put out my hand, and touched the face of God.

In memory of F.L. John Sterne, D.F.C.... and all those who paid the ultimate sacrifice.... We Will Remember Them !!!

Bob

7
Airgun Ammo / .308 cal 106 gr. Airgun Bullet at NOE
« on: November 06, 2017, 06:28:22 PM »
Brand new design I didn't even know about, now in the store....

http://noebulletmolds.com/NV/index.php?cPath=30_515

RG2 and RG4 moulds available, they come with flat nose (106 gr.), cup point, and hollow point pins (100 gr.)....

Bob

8
Ask Bob / How Barrel Harmonics Can Affect POI
« on: November 06, 2017, 03:30:41 PM »
There have been some recent discussions about the use of Harmonic Barrel Tuners to dial in a PCP for a given pellet and velocity.... Here are some generic charts showing what happens when the pellet arrives at the muzzle of a barrel that is vibrating in the vertical plane.... The first chart shows an example of the angle the muzzle is pointing, relative to the boreline at rest when the trigger is pulled.... I drew the chart so that a pellet travelling at 900 fps would arrive at the muzzle at the point where the barrel is at its highest.... with that being a bit over 2 MOA (it might be more or less, depending on barrel stiffness)....



You would think that would be the worst possible case, because the barrel is the furthest from its position at rest.... but in fact it is pausing at that moment, because is it reversing direction.... In the second chart (a detailed view of the last peak in Chart 1), each dot represents the time the pellet arrives at the muzzle, with a dot every 10 fps from 800 to 1000 fps.... 900 fps is at the peak, because I chose the vibration frequency to make it so.... You will note that there is hardly any difference in the angle the pellet leaves the barrel for quite a large ES either side of 900 fps.... That intuitively might tell you that is the optimum velocity for a barrel vibrating at this frequency.... However, that is incorrect also....



The next chart shows the POI of the pellet (I chose a BC of 0.036 to get these numbers at the various velocities).... If the gun is sighted in at 100 yards when the velocity is 900 fps, then at 800 fps it hits about 6.5" low, and at 1000 fps about 4.2" high.... At 800 fps, a 20 fps ES changes the POI about 1.5".... At 900 fps, it makes about a 1.0" difference.... and at 1000 fps, that 2% ES changes the POI only about 0.7"....



When you combine the flatter trajectory of faster shots with the angle of the muzzle at the instant the pellet leaves the barrel.... you get an interesting result, as shown below (gun again sighted in at 900 fps at 100 yds.).... The slower pellets (eg. @ 800 fps), arriving after the peak of the barrel vibration, shoot lower on the target.... but the faster (1000 fps) pellets, arriving when the barrel is still travelling upwards, ALSO shoot lower on the target.... This increases the vertical dispersion for pellets travelling slower, and decreases it for pellets travelling faster.... Now, for that same 20 fps ES, pellets at 800 fps have vertical stringing of 2.1" (instead of 1.5").... those at 900 fps are still about 1.0".... but pellets at 1000 fps, with that same 2% ES, only have a vertical stringing of less than 0.3" (instead of 0.7")....



It is important that you do not interpret this to mean that faster is always better.... The concept I want you to understand is that by using a movable weight (or changing a weight) attached to the muzzle, you can change the frequency at which the barrel vibrates.... If you want to shoot at 900 fps, then you want the barrel to be on the upswing, but nearing the top of it's movement, when the slowest pellet arrives at the muzzle.... This adjustment is arrived at by trial and error, but can reduce the vertical dispersion of your groups once you find it....

Think about it this way.... The slowest pellet in your shot string arrives at the muzzle when the barrel is nearing the highest point on the swing, but still on the way up.... It is launched on a higher angle, but going slower, drops more.... Conversely, the fastest pellet arrives earlier in the vibration cycle, when the barrel isn't pointing quite as high, but going faster, drops less.... The combination of these factors can reduce the vertical stringing of the shots when they impact a target downrange.... I hope this explanation helps you understand the merits of tuning the harmonics of your barrel for the velocity you are using....

Bob

9
Airgun Ammo / NOE Sale
« on: November 04, 2017, 07:28:51 PM »
All of NOEs Moulds .284 cal and smaller are currently on sale at 15% off.... Offer expires 11/6/17 at 11:00 PM MT, Can be combined with your 1000 dollar club discount. Sale price applies to inventory on hand only. Not valid on previously placed orders.

Bob

10
Ask Bob / What Effect Does Dwell Have
« on: November 03, 2017, 02:45:52 PM »
There seems to be a fair amount of confusion about the effect of valve dwell in PCPs.... I thought I would take the time to plot some interesting charts using Lloyd's Internal Ballistics Spreadsheet for what happens inside a "typical" PCP.... I chose a .25 cal with a 24" barrel operating at 2000 psi (regulated), using a 50 cc plenum, with a fairly high efficiency of 75% (which can be reached with large ports).... I used a typical "wasted" transfer port volume, and pellet bore drag of 1 lb. and breakaway drag of 2 lbs.... I included the air mass in the calculations, and used Isothermal expansion while the valve is open and Adiabatic expansion after it closed.... You don't have to know all that stuff, just realize that I tried to choose typical conditions, to produce GENERIC charts.... The absolute values will not be valid for any particular PCP, but the RELATIVE things that happen should be.... I used two pellets, the 25.4 gr. JSB Kings and the 34.1 gr. King Heavy MkIIs.... Here are the charts....



In the first chart, above, the blue lines represent the velocity (fps, on the left axis), and the red lines the energy (FPE, on the right axis).... The solid lines are for the 25.4 gr. pellets, and the dotted lines are for the 34.1 gr. pellets.... and all are plotted against the valve dwell, in milliseconds (mSec).... Note that both pellets have equal FPE (~ 62.3 FPE) at a dwell of 1.8 mSec....




In the second chart, above, the black lines represent the pellet position (left axis), from breech (0%) to muzzle (100%), when the valve closes.... and the green lines represent the efficiency (FPE/CI, on the right axis).... Again, the solid lines are for the 25.4 gr. pellets, and the dotted lines are for the 34.1 gr. pellets.... and again, all are plotted against the valve dwell, in milliseconds (mSec).... Note that the 25.4 gr. pellet exits the muzzle in a bit under 3.0 mSec, and the 34.1 gr. pellet in about 3.3 mSec.... therefore any additional dwell simply wastes air, as it cannot add more velocity and energy to the pellet....



The table, above, compares the two pellet weights at 1 mSec (green), 2 mSec (blue), and 3 mSec (red).... You can see what happens to the pellet position at valve close, muzzle velocity and energy, and the efficiency, at those three dwells.... At 1.8 mSec, when the FPE is equal (62.3 FPE) for both pellets.... the 25.4 gr. has travelled about 42% of the way to the muzzle, but the 34.1 gr. only about 33%.... Even though the valve dwell is identical, since the lighter pellet has travelled further down the barrel, more HPA has been released by the valve (to fill the barrel behind the pellet).... This means that more air was used to produce the same energy.... so the efficiency with the lighter pellet was less also.... While the 34.1 gr pellet achieved 62.3 FPE at 1.21 FPE/CI.... the 25.4 gr., at the same energy, only managed 1.00 FPE/CI.... In fact, the 34.1 gr. pellet is more efficient than the 25.4 gr. at all valve dwells....



The last table, above, shows what happens if you close the valve when the pellet is halfway along the barrel.... The dwell is adjusted so that both pellets are at the 50% point when the valve closes....  The % fps and % FPE numbers show the loss compared to the maximum possible, and the % air numbers show the amount of air used compared to running flat out.... Note that closing the valve at halfway saves over 40% of the air, while losing less than 5% of the velocity and 10% of the energy.... That is why I have been saying it's not worth tuning for the last few fps, it simply costs way to much air to get them....

I hope that this post gives you some insight into what is happening inside the barrel.... I will leave this thread unlocked, providing you stay on topic with your questions....

Bob

11
Ask Bob / A Decade Passes
« on: November 03, 2017, 08:42:07 AM »
10 years ago today I made my first post on the Canadian Airgun Forum.... When I got back into airguns I had no idea how all-consuming the Hobby would be.... or what I would be doing a decade later.... It has been a pleasure to be a part of all the Forums and I hope to continue contributing, and learning, here for a long time to come....

Bob

12
Airgun Ammo / .357 cal 136 gr. BBT at NOE
« on: October 23, 2017, 12:25:49 PM »
This Group Buy has moved to last call at NOE....

http://noebulletmolds.com/smf/index.php/topic,1629.0.html

Bob

13
Airgun Tanks, Compressors and Filling / Compressor Explosion
« on: September 20, 2017, 12:33:32 PM »
No idea of the make or model, or what caused the failure.... but this HPA compressor failed big time....





Anyone recognize the type?....

Bob

14
Airgun Ammo / Hybrid Bullet / Pellet Poll
« on: September 19, 2017, 08:23:43 PM »
This poll is to find out what weight you would like to see for a bullet such as is being discussed in this thread.... Only 1 choice in the poll to force you to choose your most preferred weight....  :P

http://airgunguild.com/ask-bob/hybrid-pelletbullets-possible/

Here is a preliminary concept drawing.... These would have to be cast, it is the molds that would be available, although if we got a successful design we could look into having them produced, of course.... right, Travis?....



There are a lot more people with .25 cal airguns, so it seems logical to start there.... The concept, if successful, could be scaled up (or down) for other calibers, of course.... Shorter, lighter bullets would require a redesign of the nose.... it would need to be "blunter" to keep a long enough bearing area.... either a larger Meplat, or a smaller Ogive Radius, or both....

Bob


15
Heavier pellets will generally produce slightly higher efficiency, particularly if you don't change the hammer spring preload.... They end up going slower, but the valve dwell is still the same, so the valve closes before the pellet has moved as far down the barrel.... That leaves more length for the air to expand, so more of the energy in it gets extracted, and the residual muzzle pressure is lower, resulting in less report.... In addition, slightly less air is released from the valve, because the length of barrel filled while the valve is open is less.... Since the heavier pellet often has higher FPE as well, from that extra energy extracted from slightly less air, the efficiency increases.... This makes the gun quieter, with more FPE, with the heavier pellets.... and often gains you a shot or two as well....

Bob

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