Bullet acceleration traveling in the bore

Culpeper

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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

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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).
 
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Sheldon N

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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

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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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TiroFijo

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Check any good ballistic program, a bullet fired from a level barrel takes MORE time to drop compared to one that is simply dropped. This is due to the vertical component of its vastly larger drag.

Also, a bullet does not leaves the barrel "slightly unstable" and then stabilize down range. The bullet is dinamically stable but always suffers to some degree from a wobbling motion due to second degree effects (precession and nutation, etc.), and these effects damp out downrange.
 
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bax

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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?
Really interesting question. We have all seen the high-speed photos where some gas escapes from the muzzle before the bullet emerges - so there is some leakage around the bullet while it is in the barrel. I wonder if that extra hot gas in the barrel is significant?

The rifling is impressed into the jacket but does the jacket ride up the barrel on a layer of gas or is the jacket being scraped off the entire way? Since the bullet is traveling faster than pressure waves travel in air, the air in the bore is pushing back as the bullet is being shoved up the bore. Given the mass of that air, I expect - that is, I guess - that has no noticeable effect on the bullet.

The bullet rotates in the barrel because of the rifling and the lead core is captured inside the jacket so it rotates with the jacket, does the nose stay straight ahead or does the bullet nose rotate in a circle like it does after leaving the muzzle or does it cant off to one side or something? Put another way, does the instability that the bullet demonstrates in the air start in the bore or once the bullet is in flight?

I would expect - that is, I guess - that the bullet does not accelerate once the expanding powder gas stops pushing it. I'm not sure where that happens - right at the muzzle, 5mm past, a foot? No idea.
 

LastShot300

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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?
Exterior ballistics does not account for these effects which are basically non existent, it's the realm of interior ballistics
 
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Culpeper

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I beg to differ that forces in the bore are irrelevant. Wouldn't the bullet be travelling on air due to compressed gas both behind and in front of it at least at some point? If so, something sure as Hell is happening inside the bore when it goes supersonic and damn quick too. If we are able to measure deceleration from MV to the target then why can't we measure acceleration from the bullet to the muzzle? Also, surely you can see where I'm going with this. The so-called ill defined and often misused words, harmonics (frequencies) and it's associated cause and effect accuracy nodes (intersections). So, it is relevant to exterior ballistics even though this part is the short life of interior ballistics leading up to it.
 
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W54/XM-388

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Really interesting question. We have all seen the high-speed photos where some gas escapes from the muzzle before the bullet emerges - so there is some leakage around the bullet while it is in the barrel. I wonder if that extra hot gas in the barrel is significant?.
There is a good chance what you might also be seeing to at least some extent is the solid column of air that filled the entire volume of the barrel at standard pressure from the muzzle to where the bullet was, suddenly being rapidly compressed/accelerated (which would heat it) and pushed out ahead of the bullet.
 
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bax

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There is a good chance what you might also be seeing to at least some extent is the solid column of air that filled the entire volume of the barrel at standard pressure from the muzzle to where the bullet was, suddenly being rapidly compressed/accelerated (which would heat it) and pushed out ahead of the bullet.
I'm referring to that cone of burning gas that extends 3 to 5 bullet lengths from the muzzle as the bullet emerges from the muzzle. I agree that the air in the barrel gets compressed but I think (that is, I guess) that the compressed air won't look like it is on fire. I was thinking about the gas that leaks around the bullet.

Interior ballistics is what is happening while the bullet is in the barrel. Exterior ballistics is what happens when it leaves the barrel. Is there any reference material for interior ballistics in small arms? Seems like there would have to be ...
 
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Bender

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I'm referring to that cone of burning gas that extends 3 to 5 bullet lengths from the muzzle as the bullet emerges from the muzzle. I agree that the air in the barrel gets compressed but I think (that is, I guess) that the compressed air won't look like it is on fire. I was thinking about the gas that leaks around the bullet.

Interior ballistics is what is happening while the bullet is in the barrel. Exterior ballistics is what happens when it leaves the barrel. Is there any reference material for interior ballistics in small arms? Seems like there would have to be ...
As soon as the bullet leaves the barrel, the hot gasses are no longer constrained by the bullet, the gasses now have “more” velocity than the projectile and the bullet, in a sense, is flying backwards as the hot gasses pass the bullet. Jim Boatright was writing about this in another thread discussing hyper stability in projectiles. In some bullet designs this could introduce some instability in a poorly stabilized projectile.
 
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bax

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As soon as the bullet leaves the barrel, the hot gasses are no longer constrained by the bullet, the gasses now have “more” velocity than the projectile and the bullet, in a sense, is flying backwards as the hot gasses pass the bullet. Jim Boatright was writing about this in another thread discussing hyper stability in projectiles. In some bullet designs this could introduce some instability in a poorly stabilized projectile.
Of course. However a little of that gas leaves the barrel while the bullet is still a few bullet-lengths down the barrel. It leaked around the bullet. I would think that gas through which the bullet flies as it leaves the barrel would behave like a turbulent fluid and introduce random instability. I would also think that a flat-based bullet would be more affected than a boat-tail. Since BR shooters commonly use flat-based bullets and often-enough get groups under a tenth, apparently this is not the case. Of course, they are shooting at either 100 or 200 yards so maybe the effect happens but there is not enough time for it to damage the group? Compromises.
 
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Bender

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Of course. However a little of that gas leaves the barrel while the bullet is still a few bullet-lengths down the barrel. It leaked around the bullet. I would think that gas through which the bullet flies as it leaves the barrel would behave like a turbulent fluid and introduce random instability. I would also think that a flat-based bullet would be more affected than a boat-tail. Since BR shooters commonly use flat-based bullets and often-enough get groups under a tenth, apparently this is not the case. Of course, they are shooting at either 100 or 200 yards so maybe the effect happens but there is not enough time for it to damage the group? Compromises.
Yes. This is what Jim was fleshing out in his research and articles.