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Thread: Powder Burn and Pressure Relationship

  1. #41
    Boolit Master
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    The reason the high expansion ratio example is less effected by barrel shortening is that by the time the bullet reaches the end of a long barrel it has already run out of steam, so to speak. If the muzzle pressure is only 5000 psi less performance will be lost than if it is 15000 psi. The low charge/high expansion example will be more efficient but not necessarily more effective than a round with high charge/projectile ratio. A lot of shooters are less concerned by efficiency than by effect. They are willing to burn a lot more powder in exchange for a modest increase in performance.

  2. #42
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    Quote Originally Posted by Larry Gibson View Post
    44man

    Another quote from Powley if I may which adds to your premiss; "a gun with low Expansion Ratio and a high ratio of Powder Charge to Bullet Weight will be affected much more by barrel shortening than will a gun of both high Expansion Ratio and low atio of Powder Charge to Bullet Weight.

    Larry Gibson
    Exactly.
    I read a lot of powley long ago but confess to forgetting so much at my age.
    You and Doc make sense.
    Common sense says powder burn rates are made for a reason. Size of grains, holes in the center, deterrents to slow ignition and burn speed, shape of grains, etc. Many powders are the exact same formulation but just changing shapes and coatings will change the burn rate.
    What is burn rate? It sure does not mean all powders go off in an inch. It is control.
    So many think it is all gone quick so look at the space shuttle. It does not get a huge blast at launch and go out. The propellent burns most of the way to orbit.
    Compare the old Bazooka with a rocket.
    The boolit is really a rocket, prolonged push until it leaves the bore. Bore length can extend or shorten push.

  3. #43
    Boolit Master
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    Increasing the powder charge does not increase the expansion ratio. It reduces it. To increase the expansion ratio one increases barrel length or decreases chamber volume.

  4. #44
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    No matter what way we look at it there is a point of no return with expansion ratio, some times called over bore, but what about being under bore?
    A 22Lr. with a 28" barrel like on a Winchester 52 certainly does not have enough powder gasses to burn the whole length of the barrel to provide a constant rate of acceleration. At some point the powder has been completely consumed, but the gases are still expanding, but at a slower rate until they reach zero pressure. In the case of the 22Lr. in a 28” barrel the bullet could even be de-accelerating due to bore friction in the last couple of inches of the barrel. This should help to make more uniform the gas pressure as the bullet is released from the muzzle.

    Unfortunately the longer barrel adds more barrel whip/nodes that can cause a greater dispersion of the bullets on the target, A chronograph could even show that this is a good load with low standard deviations and extreme spreads. (A good thing for barrel turners.)

    Barrel volume in cubic inches for a 20” barrel.
    .223 = 0.75 .243 = 0.90
    .257 = 1.00 .264 = 1.06
    .277 = 1.17 .284 = 1.23
    .308 = 1.45 .312 = 1.49
    .323 = 1.60 .338 = 1.75
    .348 = 1.86 .358 = 1.97
    .376 = 2.17 .458 = 3.24

    Basic math: For a enclosed bore, the volume is proportional to the square of the radius,
    If we double the bore diameter, the bore volume increases 4 times.

    With the above chart it shows that a larger caliber with a smaller case capacity can be more efficient with a shorter barrel.

  5. #45
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    Now we are getting somewhere.
    Yes a .22 runs out at something like 16" but a longer barrel can still make it more accurate.
    Don't depend on me for math!
    To make shorter barrels for larger and larger calibers and brass just does not work good.
    Yes, I get a 45-70 to shoot in a 10" barrel but it is with one powder and never a case full to utilize the capacity. It is just the gun and twist that makes it work. It will never be a rifle. I knew I had a handful because even a 30-30 in a 10" barrel is nuts.
    A larger case reduces pressures, that is true and I can make the 45-70 shoot as fast as a .454 with less pressure but will never reach what a long barrel can do.
    Too many believe a 10' fireball out front makes the gun more powerful. The recoil from a short gun MUST make the gun more powerful!
    Too much powder adds to projectile weight and that can reduce velocity. That is because the powder is not all gone in an inch, it is being pushed down the bore. Peak is higher from more weight but the powder can not all burn so much is ejected.

  6. #46
    Boolit Master 303Guy's Avatar
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    Yup, we're all on the same page - great inputs, thanks.

    Just a thought coming from the powder adding to projectile weight - it increases recoil! Mmm .... it may be true that the fire-ball does not mean more power but it sure does mean more fun! In the world of suppressors that higher mass and higher pressure muzzle ejecta is good for reducing recoil. Without suppressors it's bad for muzzle the unprotected ears.

    I've found load data for H4350 that indicates it does burn OK at lower pressure and I have loaded it quite low - as low as 65% case volume but with filler under a 147gr boolit.

    Anyway, something I'm surprised at is the muzzle velocity attained by my 15 inch barrelled 303 Brit pig gun. A 194gr boolit doing 2000fps using a case full of H4350. I'm still interested in increasing the bore of that gun for the purpose of getting more power out of its short barrel. Not more than 2000fps but a heavier boolit - maybe slower though. I don't really see the need to drive a heavy boolit above 1800-1900 fps other than to gain a little more 'reach' i.e. extending the point blanc range. Another perceived benefit is lower muzzle pressure which I see as a boolit de-stabling factor. Has anyone found a difference in accuracy with lower muzzle pressure versus higher muzzle pressure?
    Last edited by 303Guy; 04-06-2013 at 04:00 PM.
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  7. #47
    Boolit Master 303Guy's Avatar
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    So I put on my hearing and eye protection and tried a load of 30gr W780 with only Dacron filler (1gr) under a 206gr paper patch boolit and hoped. That's 60% load density. Well, just a soft thump was all there was. Boolit expansion was 530% and penetration into the media was 12 inches/300mm and weight retention was 100%.The primer shows a little more pressure than a no powder load i.e. not much to see. But there was some base edge feathering on one side - not good! Obviously with paper patching there was zero rifling skid. So the powder looks promising but the alloy looks too soft but OK for that velocity (I wish I knew what it was doing). I'm going to repeat it with a plain cast boolit which is lighter at 192grs. For the paper patchers, the patch came off in long strips.
    Rest In Peace My Son (01/06/1986 - 14/01/2014)

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  8. #48
    Boolit Master 303Guy's Avatar
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    Right. I repeated the test with the same boolit weight and stronger alloy and got less expansion but a similar expanded diameter but greater penetration into rubber grounds. I also did the plain cast test with a lighter boolit of 192gr and this time there was unburned powder kernels. Even so, penetration and expansion was about the same. So it seems the lower limit for W780 in the 303 case is 30grs under a 206gr boolit. One can hardly tell from the primer that the case had any powder in it. The next step is to load up a few rounds and go test them and chronograph them. That could be a while.
    Rest In Peace My Son (01/06/1986 - 14/01/2014)

    ''Assume everything that moves is a human before identifying as otherwise''

  9. #49
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    Some things work extremely well. My MOA in 7BR has a 10" barrel and would just scatter the 120 gr bullets until I tried Varget. I want the 120 gr for deer. I got it to go 2175 fps with 32.5 gr and it does 1/2" at 50 yards. It burns very clean.
    I use 4759 with a 154 gr but go to H322 for 162 and 175 gr bullets. The 175 gr load is max but will poke one hole at 100 for 5 shots. 1898 fps.
    Someday I will try Varget with the heavy bullets but there is no load info for it at all because it is not supposed to work.
    The little case is very efficient.
    Back when I had a 30-30 with a 10", only 4759 worked right, back to one powder like the 45-70. A 12" to 15" barrel would be so much easier to work, the large cases are just not right in short barrels.
    To hear of guys buying a 45-70 or .500 S&W with 4" barrels will give them too much to chew on.
    303, good to hear some of the powders you use can be reduced. That is the hardest thing to find. That scares me more then a little over max with some powders.
    The next thing we did was to use much more Pyrodex and compress it more to fit a 45-70 BPCR. That was the funniest thing ever, we had plugs of Pyrodex fly out the muzzle like flares to burn in the grass. Then we increased the charge of BP and once a certain point was reached, velocity started to drop and never stopped dropping as we added more powder. Recoil increased a lot. A 34" barrel could not burn it all.
    I don't think muzzle pressure has too much affect on accuracy if the crown and boolit bases are perfect. It is just wasted powder.
    Looking at the .22 with a long barrel where the bullet is slowing might just mean the bullet used does not want to be too fast or over spun.

  10. #50
    Boolit Master
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    I think if you look at the chart I posted in post #44, you can see how fast the bore volume can jump like comparing
    .308 = 1.45 to .458 = 3.24, the volume is more then twice meaning the pressure will be less then half with the same powder.

  11. #51
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    I think if you look at the chart I posted in post #44, you can see how fast the bore volume can jump like comparing
    .308 = 1.45 to .458 = 3.24, the volume is more then twice meaning the pressure will be less then half with the same powder.

  12. #52
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    Quote Originally Posted by Doc Highwall View Post
    I think if you look at the chart I posted in post #44, you can see how fast the bore volume can jump like comparing
    .308 = 1.45 to .458 = 3.24, the volume is more then twice meaning the pressure will be less then half with the same powder.
    Hey, you got a double post!
    Yes, very true about volume.
    I get funny thoughts like a .500 S&W with no barrel, just the cylinder or a single shot with such a short barrel the boolit is sticking out the muzzle. Fill the case with powder. There is a high giggle factor. Yet a few think a 2" .500 S&W is the ideal back up gun.
    I don't know Doc, how is it ever explained?
    According to gun writers the powder will all be burned in the case since it is longer then 1". We need to have rifled brass.

  13. #53
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    I think it depends on what you mean by "burned".

    Obviously if you are finding unburnt powder in the case or on the ground, something is wrong and it could be that the pressure drops in the barrel before all the powder has a chance to burn. Smokeless requires pressure to burn quickly, as anyone that has lit a pile of it and black powder can attest. The black will flash, but the smokeless will just burn and smolder.

    If you can hold the powder back for longer with a heavy boolit and lots of friction from the bore or case, things will improve.

    There is a point where you can still gain, eventhough it is not very efficient. Your 2" .500S&W would be a good example. Lots of unburned powder, but I guarantee that if you slide a .500JRH loaded with a max charge and the same boolit in the cylinder and touch it off it will still be slower than the .500 S&W with a max charge and the same boolit. Now, maybe not a lot, but some. And it will have taken a bunch more powder to get it done too.

    It's all relative.

    I have a .480 Alaskan on order that I think might shed some light on this once it meets a reamer.

  14. #54
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    No, I don't think the JRH will be slower and might be a lot faster.
    The thing to look at is the much larger powder charge in the S&W adding to boolit weight and raising the pressure. Less efficient use of the powder with more blown from the muzzle and more cost.
    How about a case half the size of the JRH? Would it be slower in a 2" barrel?
    I can't answer it. I have nothing to work with.
    The point of diminishing return changes with each cartridge and a whole lot higher load can mean 20 to 50 fps gain, some might reach 100 fps or more but the cost of pressure and money negates it. These are velocity advances not needed for any reason.
    Another thing not looked at is the accuracy, velocity position. A boolit needs so and so velocity and spin so if you shorten the barrel and reduce the velocity, you start to make a lot of noise only. It might be only 50 fps to turn a golden load to a scud.
    Now you need to raise the pressure in the short barrel to gain the 50 fps but find it can't be done. Now you change to a faster powder so the case capacity is of no use.
    All is give and take.
    It amuses me when someone uses Unique, 2400 or a faster powder in a 45-70. What is the huge case used for and why not a .45 Colt?
    I have two theories not to be confused with anything of course, just my ideas.
    A short barrel is better with a smaller case and a short barrel should have a faster twist rate.
    There, I let the Genie out of the bottle!

  15. #55
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    Interesting thing about 'unburned' powder is that it has changed colour so I'de assume some degree of gas release has occurred and obviously some of the powder has burned but why does the flame go out? Makes me wonder about those secondary pressure spikes on pressure traces. Do those partially burned kernels suddenly detonate - if there's enough of them and the barrel is long enough? Why do I not find them when the boolit is a little heavier? Do they simply continue burning outside the barrel after some point. There didn't seem to be much difference in velocity.

    Then I start to wonder whether there is any point in going so low in charge even if the velocity is where I want it to be when I can achieve the same thing with a faster powder like W748. I'm aiming for a gentle launch and a high muzzle velocity with most of the velocity gain taking place further down the barrel. At the end of the day only accuracy with decent velocity matters. I found it interesting that H4350 could drive a fairly heavy boolit that fast in so short a barrel. I should chrono that load in a longer barrel.

    Something I haven't done yet is burn some powder in a pan to see what the residue looks like and to compare open burn rates.
    Rest In Peace My Son (01/06/1986 - 14/01/2014)

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  16. #56
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    303, powders are just so different. Some need pressure, some just a fire. Some will be scorched and not light.
    A powder that changes color only means the coating was not broken through to reach the powder. It could be heat or pressure reduction but I will blame heat loss.
    You will not find much burning powder in a pan. The only thing you will see is some powder needs fire held longer until they light. It is the coating that retards flame.
    Even BP has a carbon coating so you need to hold a match on it for a time but then it all goes.

  17. #57
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    Powders are definitely different and using them "off the map" may give some interesting results that don't follow the rules.

    Gather up some o those "unburnt" kernels and see if you can get them to light with a match.

  18. #58
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    Quote Originally Posted by subsonic View Post
    Powders are definitely different and using them "off the map" may give some interesting results that don't follow the rules.

    Gather up some o those "unburnt" kernels and see if you can get them to light with a match.
    Yes, they burn very nicely. They light faster then new powder too.

  19. #59
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    Curious that they should light easier yet went out in the barrel when things were much hotter. Not enough oxygen under the conditions maybe? Mind you, enough oxygen will burn anything. Almost.
    Rest In Peace My Son (01/06/1986 - 14/01/2014)

    ''Assume everything that moves is a human before identifying as otherwise''

  20. #60
    Boolit Master 303Guy's Avatar
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    So I burned five powders open. Not much to learn really but they all burned completely. Shotgun powder burned cleanest but while they all burned differently with different flame appearance and slight lighting differences most left a dark ash but no partially burned kernels.
    Rest In Peace My Son (01/06/1986 - 14/01/2014)

    ''Assume everything that moves is a human before identifying as otherwise''

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Abbreviations used in Reloading

BP Bronze Point IMR Improved Military Rifle PTD Pointed
BR Bench Rest M Magnum RN Round Nose
BT Boat Tail PL Power-Lokt SP Soft Point
C Compressed Charge PR Primer SPCL Soft Point "Core-Lokt"
HP Hollow Point PSPCL Pointed Soft Point "Core Lokt" C.O.L. Cartridge Overall Length
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