Ballistics by the inch question

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Mrlucky353
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Ballistics by the inch question

Post by Mrlucky353 »

http://www.ballisticsbytheinch.com/calibers.html

Any scientific explanation as to why some calibers (357 magnum, 44 magnum) show so much more velocity gain than other calibers (9mm, 40S&W) with increased barrel length?
Archer
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Re: Ballistics by the inch question

Post by Archer »

Yeah,
The energy imparted to the bullet can be described as the area under a Pressure vs. Time (OR a Pressure vs. Length curve).

.357 Magnum and .44 Magnum typically use slower powders than typical defensive calibers: ( 9mm, 40 S&W, and .45 ACP). They also typically hold more powder than the defensive rounds. ALSO the pressure limits of the rounds are or can be higher than the defensive rounds. This means that the P vs t curve ramps up a little slower than the defensive rounds to a what may be a higher maximum P and sustains the pressure levels over a longer span of time.

For example .357 Mag has a Max pressure of 35KPSI SAME as 9x19 BUT
.357 Mag uses ~10 to 14 grains of medium or slow pistol powder for a 150 grain jacketed bullet.
9mm uses 4 to 8 grains of fast to medium pistol powder for a 125 grain jacketed bullet.

.44 Mag has a max pressure of 36KPSI vs. 35 KPSI for .40 S&W
.44 Mag uses ~15 to 21 grains of medium to slow pistol powder for a 240 grain jacketed bullet.
.40 S&W uses ~8 to 11 grains of medium to slow pistol powder for a 180 grain jacketed bullet.

.45 ACP has a max pressure of ONLY 21 KPSI or 23 KPSI for +P.
.45 ACP uses only ~4 to 11 grains of fast to medium powder for a 230 grain bullet.

More powder and a slower powder means a longer burn. In a shorter barrel a lot of that slow powder gets expended as flash and noise often burning outside the gun where it doesn't move the bullet.

The faster powder burns out faster and can be fully consumed especially in a longer barrel before the bullet leaves the barrel. That means it the pressure curve is no longer expanding but may be falling. As a result the amount of added area under the curve per inch gets smaller. In theory with a long enough barrel you might even reach the point where the friction of forcing the bullet down the barrel is more than the pressure force behind the bullet. At that point you'd actually loose velocity with a longer barrel.

That's one thing ballistics by the inch is apparently attempting to determine: where that balance point is. Problem is that point is going to vary on a PER barrel basis because of the difference in friction forces AND per ammo because of the differences in bullet weight and powder types and even primer type.

IF you look at the variation of CorBon ammo in 9mm vs. .357 you might not see quite the dramatic differences you see in most factory ammo. Part of the reason for this is that CorBon typically uses slower powder and load to +P levels with their defensive rounds.
Mrlucky353
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Re: Ballistics by the inch question

Post by Mrlucky353 »

Thanks Archer.
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Re: Ballistics by the inch question

Post by AZ Pete »

Likely, due to the slower burning powders typically used in .357 and .44 magnum loadings. Shoot some magnums, through a pistol, in the twilight hours and enjoy the muzzle flash. much less flash from the .40 or 9mm. due to faster burning powders.
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