ôMuzzle velocityö 8

[Veemax]

I have a question about the “muzzle velocity” of projectiles fired from a gun barrel! I’ve always assumed that the projectile is still accelerating- providing the rate of burn and energy of the charge is sufficient to continue to overcome friction and inertia as it leaves the muzzle- that the projectile velocity is still increasing, even when it’s left the barrel. Am I correct in saying that only the rate of acceleration decreases, ultimately of course- the velocity decreases.

I’ve read several articles in various shooting magazines, that this is not the case! They appear to suggest that the velocity decreases as soon as it leaves the barrel and that maximum velocity is at the muzzle!

Are we to say that the charge and weight of projectile are perfectly balanced- so that the velocity is constant as it leaves the last section of barrel? If this is not the case, then the projectile could still be accelerating at many tens of g’s, as it leaves the barrel.

 

[FeX32]

"velocity decreases as soon as it leaves the barrel and that maximum velocity is at the muzzle"
Yes.
How can it accelerate without any pressure behind it? (to put it simply)


Fe

 

[rb1957]

you could fire downwards, like off a mountain ...

 

[desertfox]

Hi Veemax

Once the bullet as left the barrel there's no force to push it, prior to it leaving the barrel expanding gas accelerates the bullet, but once its left everything is returned to atmospheric pressure.
Here is an online calculater you might find interesting:-

http://www.zknives.com/bali/bvtengy.shtml

desertfox

 

[Ltwine]

Max. Acc is at the instant the bullet leaving the muzzle with pressure and sudden drop of friction.

 

[handleman]

Bullet [≠] Rocket

-handleman, CSWP (The new, easy test)

 

[SomptingGuy]

This kind of question appears often in E-T forums. Max Acc is only at the intant the bullet leaves the muzzle if max force is also at that point. It's likely that max velocity is at the instant the bullet leaves the muzzle (hopefully), but velocity won't increase after it's left and the accelerating force is gone.



- Steve

 

[rb1957]

ok, i tried to have a little fun (if you fire angled downwards, there'll be a component of gravity accelerating the bullet).

but seriously, free body of bullet in barrel ... expanding gas behind it, friction acting on it, produce acceleration. as the bullet emerges from the barrel friction is removed so there'll be an increase in acceleration towards the end of the barrel. the instant the bullet emerges, the expanding gas is still (momentarily maybe) pushing the bullet (as the pressure falls to atmospheric) so there'd be another small increase in acceleration. i accept this is like "how angels fit on the tip of a needle" but ...

 

[ornerynorsk]

I have a theory that there is a slingshot effect at the moment the projectile is freed from the friction of the barrel (inertia+sudden reduction of friction). Chronometers are generally set up around 3 feet from the muzzle, so I believe any readings you get are going to be maximum anyway. Don't know if there is a method to getting an actual measurement of the projectile speed inside of the barrel. Even with high speed photography, you would not know the exact timing, in nanoseconds, that the projectile exits the casing and begins its travel down the barrel. There are milliseconds of delay from when the striker falls against the primer to when the primer actually detonates to when the charge detonates and builds sufficient pressure to put the projectile in motion.

Rocket propelled projectiles are a whole different ballgame.

 

[MintJulep]

Everyone seems to be assuming that barrel pressure is constant. I doubt that it is.

I suppose that it could either increase or decrease during the time the bullet is traveling through the barrel, dependent upon barrel length and bore and the amount and type of powder used.

 

[zdas04]

I don't think that it matters if the pressure in the barrel is constant or not. As long as there is enough greater pressure behind the bullet than in front of it to overcome friction the bullet will accelerate. At the end of the barrel, if there is still a "positive" dP then you get the "slingshot" or "spring release" effect for a final burst of acceleration, but that is a very short duration. Within a very short distance from the end of the barrel dP drops to zero and the bullet starts slowing due to friction with the air (this all assumes that the bullet was fired horizontally, any inclination will either cause positive or negative acceleration that may be greater than the negative acceleration caused by friction).

David

 

[handleman]

OK, inertia plus sudden reduction of friction cannot increase the bullet velocity. That "theory" has no grounding in any physical law.

Barrel pressure being constant is irrelevant to maximum velocity, only maximum acceleration. Maximum acceleration is not the question here.

The only question is whether the bullet continues to accelerate at some rate after leaving the barrel.

The bullet may continue to accelerate until the back end completely clears the barrel. It may accelerate slightly faster as the contact area with the barrel is reduced. However, as soon as the bullet ceases to seal the barrel, the gas propelling the bullet disperses into the atmosphere.

One might think at first that the exiting gases might continue to "blow" on the bullet for a very short distance (1-2 inches) after it leaves the barrel. I seriously doubt this is possible. Certainly not measurable.

-handleman, CSWP (The new, easy test)

 

[Ltwine]

Pop open a bottle of champaign and observe the change in velocity

 

[Ltwine]

of the cork.

 

[SNORGY]

What it comes down to is whether or not one accepts that the change in acceleration with time follows a "cusp" or a "curve" (regardless of how steep or short) from maximum acceleration to zero acceleration, and whether or not one accepts the concept of "instantaneous" energy conversion / transfer versus "extremely fast" energy conversion / transfer. If one believes the laws of classical Newtonian mechanics and the resulting algebraic expressions, then one is compelled to believe the former in each case. Therefore, continued acceleration beyond muzzle exit is only possible if there is some reserve of unconverted stored energy at that instant in time; the slingshot effect and the exiting, rapidly dissipating gas effect described above probably most accurately capture this phenomenon, since recovery and expenditure of elastic strain energy stored in the projectile would be unlikely for - for example - a plastically deformed lead bullet.

If you draw analogies, consider throwing a curling rock or pushing a child on a bicycle or pushing a bobsled. If you let go quickly at maximum push speed, it is likely the object will still accelerate further away from you, but probably what is happening is that you are giving that last little nudge or "oomph" and releasing more energy stored in your arms. If you let go very gradually, maintain your speed, and reach out, then chances are you will still be able to touch the object because it has not accelerated away from you.

Pictured that simplistically, it becomes easier to believe that instantaneous cessation of applied force produces instantaneous cessation of acceleration. The only question that then remains is whether or not it is physically possible to instantaneously commence or cease an applied force. If the bodies involved are perfectly rigid, then yes; if however there is any elasticity whatsoever, then no. The conclusion is that, since everything has at least *some* elasticity, there can be no "cusp" at all - only very very short duration, steep curves.

Coffee time.


Regards,

SNORGY.

 

[TenPenny]

Saw this on another forum:

Excepting when a bullet in in freefall, under the influence of gravity, it usually ceases to accelerate after it leaves the barrel of the gun.

The force that accelerates the bullet is the pressure built up in the chamber, and within the barrel, caused by the combustion of the propellent charge (gunpowder, to use the vernacular - although it is not normally what is traditionally known as gunpowder, which is now regarded as black powder).

Exactly where the peak acceleration happens depends on the way the propellent burns, which depends on factors such as the grain size of the propellent (smaller grains burn faster, but you can also change the shape of the grains, so the they start by burning faster, and then get slower, or visa versa). Generally, the longer the barrel, the more distance you have in which to build up speed, so you will use slower burning propellent.

In general, you want to be as efficient as you can in the conversion of propellent energy to building up speed in the bullet; so you want to make sure that when the bullet leaves the barrel, there is as little excess pressure behind the bullet as possible (as that excess pressure may make for a loader bang, but is otherwise wasted energy). Since the acceleration of the bullet will be proportional to the pressure built up behind the bullet, it therefore follows that you wish to minimise the amount of acceleration that happens just as the bullet is about to leave the barrel (ofcourse, with short barrelled guns, you may not have too much choice in the matter, but if that is the case, then you are wasting a lot of energy).

 

[SNORGY]

If what TenPenny states is true, which it probably is, then one must modify the inequality postulated by handleman:

{bullet = -rocket, bullet inside barrel}
{bullet <> rocket, bullet outside barrel}

Sorry...don't know how to do a "not equal to" sign...

Regards,

SNORGY.

 

[Ltwine]

The pressure drop at the muzzle should be small compared to drop on friction. The bullet will acc more in a very short distance then decrease. Only test can tell.

 

[cmcbain]

F=m*a dude, unless there is still gas exiting the barrel and hitting the projectile - and I suppose there is a possibility of that happening for a very short amount of time - there is no force in the equation, hence acceleration must also be zero.

 

[FeX32]

like firing downwards will give you any significant added accel...... how about we go into space, seal the barrel, take into account the effect of combustion with no O2 in space (while the GP has O2, as an oxidizer is set within the bullet casing)? ...
Why do we make such simple things so annoying for the OP.


Fe