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.

 

[MintJulep]

In the very short time while the bullet is exiting the end of the barrel friction against the barrel wall is being replace with parasitic drag from the air. This will result in a net reduction in retarding force, so assuming a constant barrel pressure, there will be an increase in acceleration as the bullet exits the barrel.

After the bullet leaves the barrel there will be some "blowing". The muzzle gas has velocity and mass - thus it has inertia. Some of the muzzle gas will be impacting on the back of the bullet, and impart some of its inertia to the bullet.

So the bullet may continue to accelerate for some small distance after it leaves the barrel. I would expect this acceleration to be very much less than the acceleration while it is in the barrel.

 

[Ltwine]

Agree with the above.

 

[ione]

When the bullet reaches the muzzle, the propellant gases have still quite high pressure (100 bar approx.). Once the bullet exits the barrel, the gases are free to move past the bullet and expand in all directions.
The gases continue to exert force on the bullet even out of the barrel until their pressure equals atmospheric pressure (this for a very short distance). Internal ballistic, intermediate ballistic and external ballistic govern the bullet motion in the barrel in the transient zone and out of muzzle.

 

[CRG]

Gee, I thought that the peak acceleration of a bullet is when/if it hits an impenetrable target

 

[aafuni]

I would immagine the acceleration out side of the barrel is neglegable and occurs over such a short distance that it really makes no difference. Part of the loud noise from a gun is due to the rapid expansion of the air out of the barrel (the expansion is almost instant and in all directions, specially those with the lease resistance ie the directions where there is no bullet to obscure the flow). For all practical cases, measuring the "muzzle" velocity even several feet infront of a gun will give you accurate velocity values. Sure this doest account for any reduction in speed due to the air in the foot or so after the barrel, but a speed measurement at this point is accurate enough for any calculation you would be doing. The uncertainty in your flight path calculations (due to bullet spin, ballance, air density, air currents etc) is going to be greater than any uncertainty in your velocity.

 

[swertel]

The not equal to sign is
=/=

--Scott

http://wertel.eng.pro

 

[rmw]

You might say that any beneficial effect of the energy of the expanding gasses behind the bullet outside of the barrel lasts about as long as the noise of the blast, maybe less.

rmw

 

[FeX32]

!=


Fe

 

[drawoh]

Gee, I thought that the peak acceleration of a bullet is when/if it hits an impenetrable target

Actually, this would be the minimum acceleration.

If the bullet is stopped in less distance than it spent accelerating through the barrel, it would experience its maxumimum absolute acceleration. This all assumes that the acceleration through the gun barrel and the target are approximately constant.

JHG

 

[Ltwine]

Is the OP still with us?

 

[CH5OH]

huh, basic thermodynamics
bullet inside barrel:
case 1ressure increases,volume increases (rapid combustion)>kinetic energy (velocity) bullet increases
case 2ressure remain constant, volume increases (slow combustion)>kinetic energy (velocity) bullet increases
bullet leaving the barrel (seal expanding gas breaks):
pressure drop,gas expanding all directions.As this will create a chock wave travelling at the speed of sound
the bullet speed when leaving the barrel is beyond that speed.So the chock wave can never give the bullet an extra push as it is to slow.

 

[Wrenchbender]

Wow, 30 responses in one day. Here’s mine: Yes, a projectile is accelerated a negligible amount beyond the muzzle. Muzzle exit pressures are somewhere on the order of ~10 ksi. When the propellant gasses are no longer restricted, they rapidly expand and outrun the projectile. For a distance of several calibers, the projectile is actually ‘flying’ backwards relative to the surrounding gasses. This is a region of aerodynamic instability and can contribute to increased yaw, which damps out quickly in well designed projectiles. Fin stabilized rounds (i.e. tank and smooth bore ammo) are more sensitive than spin-stabilized.

Projectile motion has been studied for over a century and volumes have been written. For anyone interested -

http://books.google.com/books?id=F3...6AEwAQ#v=onepage&q=gun strength curve&f=false

Some other good general references are Hayes – Elements of Ordnance, Rheinmetall – Handbook on Weaponry.

 

[CH5OH]

note on picture:
V shaped wave in front caused by bullet pushing the air asside
curly shaped wave behind the bullet caused by drag and rotation of bullet
circular shaped wave behind bullet caused by sonic boom

 

[IRstuff]

Output from an interior ballistics simulation puts peak linear acceleration at 161409 g for 7.62 mm round at 30.5 mm travel in a 609.6 mm barrel. Acceleration at the muzzle is only 17022 g. Hypothetically, one could imagine some miniscule bit of acceleration within a couple of calibers of the muzzle, but that would be about it.


The trajectory analysis showed it losing about 4 m/s in velocity with 5m of the muzzle.

TTFN

FAQ731-376

 

[CH5OH]

irstuff,
speed bullet:869m/s (refering to your attachment)
speed sound:330m/s
so no push,not even hypothetically

 

[aafuni]

CH50H,
That V shaped wave is not so much the bullet pusing the air. The V shaped wave is actually an attached shock wave.

The wave is attached for any pointed objects, and will unattach for blunt nose objects. Attached waves have alot less drag, so most supersonic rounds will have a pointed nose, rather than a round nose as seen in subsonic small calibre handgun rounds. The waves on the back side of the bullet are mach waves not shock waves (although I supose a mach wave is a type of shock wave).

The photo you have is a schleiren photo, showing the density varriations in the test fluid.

 

[SNORGY]

I am glad I read Bestwrench's post before I uploaded my simplistic viewpoint about the "blast gas" being hot and therefore likely having its corresponding speed of sound greater than the muzzle velocity of the bullet, which at least in part would permit the phenomena that Bestwrench very clearly explains and raises some doubt in my mind about the bullet being beyond the influence of the trailing shock immediately upon exiting the barrel.

Star.

Regards,

SNORGY.

 

[IRstuff]

Bear in mind that the mere fact that the muzzle pressure is well above atmospheric pressure, prior to muzzle exit, basically means that the pressure front is indeed moving at supersonic speed. That's the basic nature of explosive detonations, and that's why there's still 11000 g of acceleration at the muzzle.

Since the pressure is rapidly decreasing from its peak value, there's not a whole lot of oomph left, but there's going to be some.



TTFN

FAQ731-376

 

[ornerynorsk]

Of course there is gas still exiting the barrel at the moment of the projectile exiting the barrel. I would agree with figures somewhere around the 10ksi figure for modern shouldered centerfire cases fired from an average length barrel. If this were not so, special forces would not be using suppressors on rifles. The speed of detonation is something like 5000 m/s ? in modern small arms propellant, a little bit higher than the speed of sound, yes? With a suppressed weapon, the intended target(s) or other bystanders cannot precisely determine point of origin of the shot. The sonic boom of the bullet by itself will not tattle. Therefore, it makes incredible sense that there is still acceleration occuring at the exit of the muzzle, even if decceleration begins within an inch or so. If acceleration is still occuring, and there is the instantaneous removal of friction, voila', slingshot effect. I challenge any of you to take a new projectile and force it through a rifled barrel. There is more than a little bit of friction. This is why modern centerfire rifles are developing 50,000 + CUP's in the chamber. Anyway, I've rambled enough, someone else's turn.

 

[FeX32]

Fe