Myth-Busting Powder Burn Rate vs. Barrel Length
- Thread starter Andy
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Essentially the powder that will produce the highest MV will do so regardless of barrel length. All powders regardless of the amount or the cartridge in which it's loaded will produce higher MV with progressively longer barrels - to a point that typically exceeds 30".
The glitch in Rule 1 is "assuming the pressure is constant". It is not.
There is another way to explain this whole scenario.......If we set maximum peak pressure at say 60,000 PSI for any and all loads being discussed, then the powder which allows you to use the greatest quantity, while keeping pressure under 60K and being able to fit in the case in question, will give the highest velocity.........because the greatest quantity has the greatest available energy to expend. I too have found this with many different rifles and barrel lengths. I have loaded for 243s with barrel lengths from 18" to 26" and have found the same powders give the highest velocities regardless.
This is why I like to use the slowest possible powders in any cartridge I load, and most are moderately to highly compressed. The only powders I have found to become somewhat erratic from heavy compression have been RL 17 and RL 19, but most powders perform very consistently under moderate to heavy compression, which as we have discussed also will give the highest velocities.
Velocity is dependent on the slope of the pressure curve after peak pressure, not peak pressure itself. The longer and smoother the pressure curve the higher the velocity assuming a set peak pressure point. When I experimented with extended flash tubes I learned that peak pressure was not affected, it was in fact determined solely by the quantity of the powder charge. However velocity was affected because in essence I was reducing the initial projectile weight by the weight of the powder charge, by lighting it at the front of the powder charge. Although the peak pressure was not affected the pressure curve most definitely was. By ignition and pressure holding the powder in the case for the duration of the burn and only propelling the bullet down the bore, I managed to lengthen the curve after peak pressure and/or move the peak pressure point further down the barrel.
This is why I like to use the slowest possible powders in any cartridge I load, and most are moderately to highly compressed. The only powders I have found to become somewhat erratic from heavy compression have been RL 17 and RL 19, but most powders perform very consistently under moderate to heavy compression, which as we have discussed also will give the highest velocities.
Velocity is dependent on the slope of the pressure curve after peak pressure, not peak pressure itself. The longer and smoother the pressure curve the higher the velocity assuming a set peak pressure point. When I experimented with extended flash tubes I learned that peak pressure was not affected, it was in fact determined solely by the quantity of the powder charge. However velocity was affected because in essence I was reducing the initial projectile weight by the weight of the powder charge, by lighting it at the front of the powder charge. Although the peak pressure was not affected the pressure curve most definitely was. By ignition and pressure holding the powder in the case for the duration of the burn and only propelling the bullet down the bore, I managed to lengthen the curve after peak pressure and/or move the peak pressure point further down the barrel.
That does not change the fact that big case magnums need slower powder to produce the higher velocities. Faster powders cannot compete for velocity with a long barrel and slow powder in a big magnum. Where in the barrel the powder stops burning is irrelevant, and debatable. If the powder was all burned after 1 foot, there would never be a muzzle flash from a 24" barrel...
I don't think saying "faster powders work best with short barrels" is accurate, but it is very different to accurately state that "if you do have a shorter barrel, you really should use faster powder".
If you've got an AR15 with a 20" rifle you can use slow, medium, or fast powder.
If you've got an AR15 with a 10" barrel, it would be dumb to use a slow burning powder.
If you've got an AR15 with a 20" rifle you can use slow, medium, or fast powder.
If you've got an AR15 with a 10" barrel, it would be dumb to use a slow burning powder.
2 different things. The powder burn speed isn't limited by barrel length; peak pressure doesn't change with barrel length. Muzzle flash isn't necessarily unburnt powder.
Edit: never mind. Assume you are correct, ignore previous posters who spent a career working for powder makers. Did I mention I'm a former award winning ballerina? Astronaut, wedding planner, etc, also, just because I believe it.
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I think the powder is burt within 12 inches usually. The flash is just hot gas and glowing particles.
"Pressure is constant" is another way to say that peak pressures are the same. If they're not, then you're not making a proper comparison.
For those who refuse to accept what's in Post #1, as I said earlier:
I have on numerous occasions found unburnt powder in my barrel and come to the conclusion that flash at the muzzle is due to unburnt powder, since in my books the correct burn rate is something that stops pushing the bullet right about the time it clears the muzzle anything more and it falls into the overbore capacity.
I have also found unburnt powder on the ground in front of the muzzle during winter shoots, the granules of powder being clearly visible on the snow surface.
I have found this to be true for both low and high expansion cartridges. ( and especially prevelant with muzzleloaders)
Therefore the comment that all the powder is burnt in the first 1 to 4" of barrel is not correct in my books unless we are limiting this to very specific conditions.
I believe there are too many variables to make blanket statements that the powder that gives the highest velocity will do so weather it is from a long or short barrel even if in most cases that may be so, I don't think it is an absolute.
BB
BTW thanks for posting this most interesting topic Andy
I have also found unburnt powder on the ground in front of the muzzle during winter shoots, the granules of powder being clearly visible on the snow surface.
I have found this to be true for both low and high expansion cartridges. ( and especially prevelant with muzzleloaders)
Therefore the comment that all the powder is burnt in the first 1 to 4" of barrel is not correct in my books unless we are limiting this to very specific conditions.
I believe there are too many variables to make blanket statements that the powder that gives the highest velocity will do so weather it is from a long or short barrel even if in most cases that may be so, I don't think it is an absolute.
BB
BTW thanks for posting this most interesting topic Andy
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Two great references that document tests conducted with various burn rates of powders and cutting a barrel 1" at a time from 30" to below 20" are found in:
- American Rifleman Feb and Mar 1950; and
- Handloader Issue 243 - Oct-Nov 2006.
I have copies of both, but can't post for copyright reasons, and besides, they're too large (>25 MB in PDF, >90 MB in TIFF) anyways.
- American Rifleman Feb and Mar 1950; and
- Handloader Issue 243 - Oct-Nov 2006.
I have copies of both, but can't post for copyright reasons, and besides, they're too large (>25 MB in PDF, >90 MB in TIFF) anyways.
Powder does not contain enough oxygen to burn all the propellant. So propellant is always expelled at the muzzle. Might be a solid, might be in gas form. If gas, it sometimes ignites when it hits the air (oxygen). This flash is a foot or so in front of the muzzle. It might also produce an extra loud bark of its own.
Some powders (most) contain additives to reduce muzzle flash. Some to a better job then others.
Muzzle flash has nothing to do with unburned powder or lost energy.
Well there you go. Everything I and others say on the subject debunked by the Quickload Software program.
I guess since I use a lot of US869, I better look for a source for 48" barrels!
P.S. I've done quite a bit of load development with a 6.5-06 with a 22" barrel. You'll get max MV's with Re26 and 140 gr bullets, and US869 and 155 gr bullets regardless of barrel length.
Another way to look at it:
At 65,000 psi (limit for 6.5-06 A-Square) Inputted a 40" barrel for illustration purposes.
H-1000 63.62gn all consumed 29.5" for a velocity of 3432 ft/s, 95% consumption 13"
H-4831 60.10gn all consumed 33" for a velocity of 3434 ft/s, 95% consumption 12.5"
H-4350 56.14gn all consumed 21.5" for a velocity of 3364 ft/s, 95% consumption 9"
At 65,000 psi (limit for 6.5-06 A-Square) Inputted a 40" barrel for illustration purposes.
H-1000 63.62gn all consumed 29.5" for a velocity of 3432 ft/s, 95% consumption 13"
H-4831 60.10gn all consumed 33" for a velocity of 3434 ft/s, 95% consumption 12.5"
H-4350 56.14gn all consumed 21.5" for a velocity of 3364 ft/s, 95% consumption 9"
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Looking at post #36 the Faster H-4350 will be harder on the throat as 95% of the burn occurs in the first 9"
Andy and Ganderite are correct and if you have Quickload it will tell you the percent of powder burned and why you have more muzzle blast and flash.
I'm posting the links below because I have a Savage .308 Hog Hunter with a 20 inch barrel.
308 Winchester / 7.62x51mm NATO: Barrel Length versus Velocity (28″ to 16.5″)
https://rifleshooter.com/2014/12/308-winchester-7-62x51mm-nato-barrel-length-versus-velocity-28-to-16-5/
Short barrel 16.5 inch 308 Winchester load development
https://rifleshooter.com/2017/01/short-barrel-16-5-inch-winchester-load-development/
I'm posting the links below because I have a Savage .308 Hog Hunter with a 20 inch barrel.
308 Winchester / 7.62x51mm NATO: Barrel Length versus Velocity (28″ to 16.5″)
https://rifleshooter.com/2014/12/308-winchester-7-62x51mm-nato-barrel-length-versus-velocity-28-to-16-5/
Short barrel 16.5 inch 308 Winchester load development
https://rifleshooter.com/2017/01/short-barrel-16-5-inch-winchester-load-development/
so I would go with the slowest powder for a bolt gun , that gives the best accuracy, and save my barrel.
Great point! Something I for one hadn’t really considered before as a variable in chosing powder. IMHO, and further proven in the actual data provided in this thread, there’s no point in chosing a radically faster or slower burning powder based on barrel length. Except in the case of “rifle caliber” pistols. To me, all the published load data out there puts pistol, rifle, and shotgun powders in the caliber ranges where they belong. Based on actual testing and product development. With the exception of radically shorter barrels.
