Bullet Velocity Question


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XD Fan
June 19, 2010, 08:02 PM
How far does a bullet travel before it reaches maximum velocity and begins to decelerate? Are we talking about a few inches from the muzzle? a few feet? a few yards?

I realize that the answer to my question will vary from gun to gun and from caliber to caliber. I am just interested in a rough answer.

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FatPants
June 19, 2010, 08:22 PM
As soon as the bullet leaves the muzzel it begins to slow down.

XD Fan
June 19, 2010, 08:25 PM
I thought that might be the case but I wasn't sure. Thanks, FatPants.

Dave A
June 19, 2010, 08:26 PM
A bullets maximum velocity occurs the moment it leaves the muzzle, thereafter air drag causes it to slow down. The exception to this would be if the bullet is fired at a barrel elevation that causes the bullet causes the bullet to slow down due to the effect of gravity then accelerate as it falls to the target. If a bullet is fired strait up it will at some point in time achieve zero speed at the peak of trajectory then speed up as it falls. Therefor bullets can both slow down and speed up after leaving the muzzle. An intresting aside to this fact is that the theoretical ideal barrel elevatiion is 45 degrees absent an atmosphere: actual working elevation is about 37 degrees due to the non ideal parabolic curve induced by atmospheric drag. Large artillery can fire at barrel elevations as great as 53 degrees and take advantage of reduced drag at the peak of the arc, air is way thinner 36000 feet, an altitude acieved by the real big guns.

MachIVshooter
June 19, 2010, 09:06 PM
Except for very specific cases (like Aguila Kolibri's, which actually slow down in the barrel), the highest velocity attained will be the instant the bullet leaves the muzzle.

tack
June 19, 2010, 09:30 PM
For the 22 long rifle bullet, the maximum velocity is achieved with a 14 inch barrel. Precision shooters use a longer barrel for accuracy, not velocity. (The puff of gas that expands around the bullet as it leaves the barrel causes some inaccuracy.) I've often thought they could have the best of both worlds in a supersonic round if they used a ported barrel.

MachIVshooter
June 19, 2010, 11:46 PM
For the 22 long rifle bullet, the maximum optimum velocity is achieved with a 14 inch barrel.

There.

They continue to accelerate in longer tubes. But the velocity gain per inch is at the point of diminishing returns after 14"-16". They won't begin to decelerate in the barrel until you're well beyond 30".

Mal H
June 20, 2010, 12:28 AM
Right MachIVshooter. It has been said over and over on the 'net that a .22 (any size - Short, Long, Long Rifle) will start to slow down in a long but standard length barrel (> 16" <= 24"). That is false and has been proven to be so many times.

wishin
June 20, 2010, 02:44 PM
the highest velocity attained will be the instant the bullet leaves the muzzle.

They continue to accelerate in longer tubes. But the velocity gain per inch is at the point of diminishing returns after 14"-16". They won't begin to decelerate in the barrel until you're well beyond 30".


I'm confused; which one is it?

MachIVshooter
June 20, 2010, 03:05 PM
I'm confused; which one is it?

It's both. Except for finding some truly obscure creation, no gun out there is going to have a barrel so long that the bullet fired from a cartridge of it's primary chambering will actually decelerate before it clears the muzzle. When was the last time you saw a .22 rifle with a barrel in excess of 30 inches?

In a case such as that with a .22, the powder may be completely burned in the first ~15", but that doesn't mean the bullet has stopped accelerating. Peak velocity does not occur simultaneously with peak pressure. Deceleration will only happen once the force (pressure) behind the bullet is less then the drag from the bore and atmosphere in front of it.

wishin
June 20, 2010, 09:07 PM
Thanks. How's the velocity measured in the barrel? One would logically expect that the friction the bullet encounters in the barrel would slow it down, however negligibly.

XD Fan
June 21, 2010, 07:07 AM
Wow, thanks for the great answers.
I do have one more question now. Can someone explain the following:

"But the velocity gain per inch is at the point of diminishing returns after 14"-16"."

Does this mean that this is the point at which the acceleration rate slows? Why would this be a problem as long as the bullet is continuing to accelerate?

1911Tuner
June 21, 2010, 07:27 AM
I've written this up a couple times in the past, but it's worth doing again.

A few years back, a gun magazine writer decided to address the question of velocity and barrel length. In order to keep it allapples to apples, he started with a single .30 caliber rifle with a 26-inch barrel, and lopped it off in one-inch increments with several small lots of handloaded ammunition. Same bullet, same primers, and same cases. Only the powders changed. He used a near maximum listed charge from one loading manual for each powder. The powders were chosen by burn rate, and the powders used were all taken from the list of acceptable powders for the caliber.

He discovered that one of the faster powders in the group didn't follow the old "Longer barrels give higher velocities" rule once a certain barrel length was passed. If memory serves me, it were IMR 3031. Top speed was reached at 23 inches of barrel, and there was very little difference at 22 inches. At 24 inches, the velocity was actually a bit less than it was at 23 inches...and less again at 25 and 26 inches, though the difference from 24-26 was negligible.

Officers'Wife
June 21, 2010, 07:42 AM
Common sense tells you that when the powder stops burning the ball will stop accelerating and slow down. My late uncle would experiment with 'corning' black powder in attempt to get higher velocity out of his homemade BP rifle, a 45 caliber with a 52 inch barrel. His corn of choice was 45 X 33 cylinders of powder with a perforation completely through the cylinder. As the inner core of the cylinder burned it increased the surface area of powder available causing an increase of pressure as the ball moved down the barrel.

Further proof that a little knowledge is not nearly as dangerous as a curious man with far too much time on his hands.

Sport45
June 21, 2010, 10:08 AM
When the powder stops burning it has generated all the gas it is going to and will "probably" be at peak pressure. That gas continues to expand and push the bullet down the barrel until it leaves (or gets stuck :(). Because the gas pressure is decreasing after the peak it makes sense that the acceleration rate will decrease. The bullet is still accelerating, it's just not gaining speed as quickly as it does at peak pressure.

There may be a millimeter (maybe) once the bullet has left the barrel and is no longer affected by bore drag, where the expanding gasses behind it my give it a little "kick". In my mind, this would explain why the rifle's crown and the bullet's base are so important to accuracy.

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