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Old July 16, 2000, 03:21 PM   #1
JackFlash
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We all agree that bullets decelerate down range.

The question then is: Do bullets reach terminal velocity at the muzzle? Or are they still accelerating after they leave the muzzle.

Somewhere I seem to remember that bullets keep accelerating after they leave the muzzle. Has to do with being free of the friction in the bore and the physics of getting any mass to overcome inertia and accelerate.

Let's not just speculate on this one. Let's try to come up with some authoratative citations. This should be a good topic of conversation at the next pool party.
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Old July 16, 2000, 03:35 PM   #2
Southla1
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According to some of my older manuals and the internal ballistics sections they do not. The reason I am saying this is that some actualy start to decelerate in the barrel due to bullet friction over coming the expansion of the gases. This is using too slow a burning powder of just the makeup of the round. If I am not mistaken with a .22 LR the optimum barrel length is 16 or 17 inches. I can see no way one can accelerate with not gases behind them to help "shove" plus air resistance is acting on the bullet BEFORE it leaves the barrel. I do know this if a bullet is chronographed at 5 feet (this is too close with some large capicity cases) the 10 feet then 15 feet etc. it is slower the farther you get from the muzzle. I have done this with a .22 so if it is accelerating it is doing it in the first 5 feet because after that is is slower and slower etc.

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[This message has been edited by Southla1 (edited July 16, 2000).]
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Old July 16, 2000, 03:58 PM   #3
JackFlash
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This is what I turned up in an initial search of web sites:


FIELD MANUAL
NO. 6-40
MARINE CORPS
WARFIGHTING PUBLICATION
NO. 3-1.6.19

HEADQUARTERS
DEPARTMENT OF THE ARMY
U.S. MARINE CORPS
Washington, DC, 23 April 1996

FM 6-40
Tactics, Techniques, and Procedures for
FIELD ARTILLERY MANUAL CANNON GUNNERY


3-2. Transitional Ballistics

Sometimes referred to as intermediate ballistics, this is the study of the transition from interior to exterior ballistics. Transitional
ballistics is a complex science that involves a number of variables that are not fully understood; therefore, it is not an exact
science. What is understood is that when the projectile leaves the muzzle, it receives a slight increase in MV from the escaping
gases. Immediately after that, its MV begins to decrease because of drag.

[This message has been edited by JackFlash (edited July 16, 2000).]
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Old July 17, 2000, 01:25 PM   #4
Southla1
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<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by JackFlash:
What is understood is that when the projectile leaves the muzzle, it receives a slight increase in MV from the escaping
gases. Immediately after that, its MV begins to decrease because of drag.
[/quote]

That would make sense seeing that the bullet was still being shoved down the barrel by the expanding gas and at the instant of the bullet clearing the muzzle the barrel friction is gone, hence the gas has an easier job.

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Old July 17, 2000, 01:40 PM   #5
bfoster
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Southla 1... A barrel length at which 1- the combustion of the powder is complete & 2- returns in velocity start to diminish won't always be optimum. "Shootability" of a rifle to be used in the field has to be a compromise of weight, balance, moment of inertia, barrel length, action type, how stocked etc.

If the gas is still rapidly expanding at say 16" in a 22 L.R. (it is) the pressure is considerably higher at the bullet base than is atmospheric pressure. The bullet will continue to accerlerate, but at a reduced rate compared to the rate of acceleration at say 8". After engraving and the bullet attaining rotational velocity ( which happens very quickly, in the first inch of bore), frictional losses are very small, as are losses due to the bullet being heated.

I've not calculated the length barrel at which a 22 L.R. will start to decelerate due to those factors accociated with barrel length, but it is longer than a practical rifle can be built. This length for one powder & bullet combination in the 50 BMG is on the order of 40 feet., for the 6 PPC, 30 feet. (A modified form of the Powley equations was used here).

Bob


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Old July 17, 2000, 01:42 PM   #6
bfoster
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Sorry about the double post.

Bob

[This message has been edited by bfoster (edited July 17, 2000).]
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Old July 17, 2000, 01:49 PM   #7
bfoster
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JackFlash... Harold Vaughn, formerly of Sandia National Labs, found that this is so. See his book, Rifle Accuracy Facts, Precision Shooting, 1998.

Bob
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Old July 17, 2000, 05:29 PM   #8
johnwill
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From all I've read, most of the acceleration takes place in the barrel. The bullet can get a small "boost" upon exit, but I don't think it's all that significant. I'd have to think that if there was significant acceleration after the exit, you'd lose accuracy too.
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Old July 17, 2000, 06:15 PM   #9
Ruben Nasser
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The "transition zone" from interior to exterior ballistics in hadguns calibers is very small (2"-4"?), and added acceleration almost nill. This can be found calculating the forces acting on the bullet at the moment it exits: gas pressure times bullet diameter is higher at the back than aerodinamic drag calculated for muzzle velocity, and this drag is not even real (it's very difficult to calculate) because of the forward gas flow around the bullet (the gas moves faster than the bullet). After leaving the muzzle the pressure drops so fast it can not accelerate the bullet. When you have a much larger gas flow (rifle magnum calibers, 50 BMG, artillery, etc.) this distance increases, and this gas flow can make a few fps increase. If you have a muzzle brake you are redirecting this flow and losing this few fps.
bfoster is right about bullets slowing down VERY slowly inside the barrel, and I've read articles by Bob Forker and other technical gunwriters reaching the same results from experiments.
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Old July 17, 2000, 06:45 PM   #10
Southla1
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There was an aritcle in, if I am not mistaken, The American Rifleman, many years back (1973 or so)about the barrel lenghts, and velocities. The 22 was at 16 inches optimum. When I stated optimum length I meant optimum for velocity generated not handling or hunting or any other purpose....strictly dealing with velocity. Anything longer than that length the velocity was remaining the same or actually diminshing. If I remember correctly the 30-06 dealt with in the same article was on the order of 28 feet I think. Whoever did the testing actually was using ultra long barrels in a pressure gun and choping off a foot or so at a time, firing and chronographing the results. Getting back to the 22 I am not sure if it was a .22 Short, .22 Long (do they still make them?) or .22 LR. I have not seen .22 longs on the shelf for years, of course they are nothing more than a .22 Short load in a LR case so it would make sense not to make them anymore.

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Old July 17, 2000, 06:46 PM   #11
Southla1
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There was an aritcle in, if I am not mistaken, The American Rifleman, many years back (1973 or so)about the barrel lenghts, and velocities. The 22 was at 16 inches optimum. When I stated optimum length I meant optimum for velocity generated not handling or hunting or any other purpose....strictly dealing with velocity. Anything longer than that length the velocity was remaining the same or actually diminshing. If I remember correctly the 30-06 dealt with in the same article was on the order of 28 feet I think. Whoever did the testing actually was using ultra long barrels in a pressure gun and choping off a foot or so at a time, firing and chronographing the results. Getting back to the 22 I am not sure if it was a .22 Short, .22 Long (do they still make them?) or .22 LR. I have not seen .22 longs on the shelf for years, of course they are nothing more than a .22 Short load in a LR case so it would make sense not to make them anymore.

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