Bullet acceleration traveling in the bore

Culpeper

One divided by F
Nov 25, 2006
2,167
637
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Roswell NM
#1
How quickly does the bullet go from transonic to supersonic before it exits the muzzle? Does it occur rather quickly and then begins deceleration before MV or is there a slight increase in speed once it is free of the bore? What is happening between the bullet and barrel during the speed of sound transition? Same question once it becomes supersonic?
 

Apnea

learning
Sep 17, 2017
249
72
28
CA
#2
My thinking is that with a "normal" cartridge and barrel combination there is enough propellant to accelerate a bullet until it leaves the barrel, at which point you've got all of the velocity you are going to get. No further acceleration can occur due to conservation of energy (can't get velocity for free). While it might be that case that what I would call the muzzle blast continues to briefly aid the bullet over a very short distance clear of the bore, I think those effects would be negligible and safe to ignore for most purposes, although we know an ugly crown can affect the bullet (I'm guessing due to turbulence in the muzzle blast).
 

Sheldon N

Blind Squirrel Finds a Nut
Sep 24, 2014
2,745
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Pacific Northwest
#3
Happens in the first two inches of the bullet travel. Acceleration continues until it exits the muzzle, but the rate of acceleration slows the farther the bullet travels (as pressure behind the bullet decreases). Supersonic inside the barrel means nothing really, there is no shockwave because the bullet is fully constrained by the barrel walls. I doubt there's any meaningful acceleration once the bullet exits the muzzle, since pressure basically falls to zero as soon as the seal is broken.
 

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Strykervet

Resident Phoenix Eye and Dim Mak Instructor
Jun 5, 2011
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Pierce County, WA
#4
The bullet accelerate the moment the primer fires --as long as it's moving faster and faster, it's still accelerating. Most barrels are made to do this, seldom does someone build a rifle with a barrel so long he gets diminishing returns, let alone one so long that exhausts every joule of spent gas right up to the muzzle. It's impractical. So you can assume that pretty much ALL barrels accelerate the bullet all the way through.

But the moment it leaves the bore, it begins to fall and to decelerate. This happens the INSTANT it leaves the barrel. The cosine element will continue on according to Newton in a straight line until affected by elements such as wind. This is your deceleration. The sine element will reduce height of the bullet with respect to the ground at 9.8m per second per second. This is gravity. If you fired a bullet from a level barrel and dropped one at the same time it left the bore, and ground was same height all the way through, then the two bullets will hit the ground at exactly the same time. Fact.

How you analyze a bullet in the bore is different from how it's done in the open. It's internal vs. external ballistics.

But there is no sub/trans/super transfer really going on inside the bore. Basically the gas pushes from behind overcoming the friction of the bullet riding the grooves and lands. It's continuous but it's not a linear transition, more logarithmic I'd assume. This happens in all barrels. The rifling equation takes into consideration the muzzle velocity of the round, so ideally the bullet should be stable as it exits regardless of it's velocity. Many pistol bullets are fired at transonic speeds but it doesn't make 'em any less accurate. Not at all. Having a bullet at 800m going 1300fps but stabilized (twist rpm) for one going 2900 at the muzzle, now that's different.

Improper rifling vs. velocity can cause bullets to exit the bore and keyhole abruptly before stabilizing or partially stabilizing or never stabilizing. Say you're using a bullet that's not optimized to the rifling in the bore. Why you should always test fire against a few sheets of cardboard real close (2 inches, six inches, a foot, two feet, etc.) before mounting a suppressor. Do subs and supers both exit stable? Round hole? .300BLK especially, but you should do all rounds before mounting a suppressor. That round has a lot of variation in bullet weight and design and forcing 'em down a short barrel with a twist that can only optimize one side of the spectrum, well, it just makes sense to check it out beforehand to be sure keyholing isn't a problem. Also, a damaged crown can cause the bullet to exit funny if the gas doesn't exit uniformly. Some cans that index TDC will actually have an elongated oval hole in the blast baffle, and I can only assume it's to allow the bullet to stabilize if need be in that critical couple inches.

Some bullets take longer to stabilize after leaving the bore, I've heard that about 175gr.SMK .308 @ 100m or less. Don't know how true it is. Then some exit the bore slightly unstable and stabilize rather quickly. The only way to tell if it's unstable at the muzzle is the cardboard test.

I hope that answers your question. Sorry for being long winded, I was trying to explain it so anyone could understand it. There's a lot of misconceptions about the physics of shooting and not everything is intuitive either, sometimes it just makes no damn sense.
 
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