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Posted: 12/22/2008 9:07:17 PM EDT
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Read this in the Armory Forum
Actually, the (5.56) powder is already burnt up in a 24" barrel before the bullet exits, meaning it will actually hurt velocity due to the drag of the rifling.
Is this true? |
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Okay, take a fast(ish) powder like 322.
It's all burned at 16.4" in a 20" barrel. If we move up to a 24", guess what? More velocity. And more if we go up to 30"... The gain in velocity is not a uniform curve along the different lengths, at some point you'll only be gaining about 10-15 fps per inch. The bullet drag in the barrel is an overblown myth. |
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Quoted:
Read this in the Armory Forum Actually, the (5.56) powder is already burnt up in a 24" barrel before the bullet exits, meaning it will actually hurt velocity due to the drag of the rifling.
Is this true? Kind of, increased velocity beyond the point that pressure has dropped by a third from Pmax pretty much every bit of powder that is going to burn has done so. Smokeless has a variable burn rate so when pressure drops it burns much slower (if at all in the time frame we are dealing with). 1/3 * Pmax happens within less than 5" of the barrel IIRC (rifle cartridges), but that doesn't mean the bullet isn't being accelerated. Only when the pressure decay drops beyond the point where it is no longer contributing to acceleration does the increase of barrel length hurt or fail to help an increase in velocity. BTW this is VERY dependent on powder selection, Phil Sharpe did a very interesting test on this subject back in the day (I want to say 1925 but that's off hand) in American Rifleman which involved about 20 loads of 30-06 plus reference loads and he started with a ~30" barrel and cut an inch off and retested until he was around 7". The results were all over the map some loads showed no increase in velocity beyond an 16" barrel and some gained velocity up to 26". |
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Whether powder is completely burned before a bullet exits is only part of the problem.
As long as the forced imparted on the bullet by expanding gas is greater than the friction force retarding the bullet, the muzzle velocity will increase, even after all the powder has burned. The gains are smaller and smaller as the barrel length increaseds. As barrel lengths get near 30 to 32 inches, pretty much every bit of incremental speed has been squeezed out of the gunpowder. Bullet bearing length and barrel bore smoothness are also factors. |
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Quoted: This is best illustrated by the lowly 22 long rifle cartridge. Maximum velocity is acheived using a barrel of about 16" in length, any longer and the bullet will lose velocity thru friction. Powder volume vs expansion. This problem does not plague high intensity centerfire calibers until the barrels reach an unmanageable length. I recall reading an article involving a .380 Auto cartridge, the point at which velocity was reduced through barrel length was amazing.Read this in the Armory Forum Actually, the (5.56) powder is already burnt up in a 24" barrel before the bullet exits, meaning it will actually hurt velocity due to the drag of the rifling. Is this true? kind |
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