Muzzle blast acceleration

[Archer50]

Apparently, in some powder burners, the bullet continues to accelerate after it leaves the barrel, driven by the muzzle blast - one apparently reputable website (can't find it again ) says that, as a rule of thumb, a bullet gains around 3% of its final velocity in the first few inches after leaving the barrel.

Does anyone know if this is true for airguns?

Alan

[Gavlar]

I'm going to guess that it must do, though the effect would likely be nominal. Would the use of an air stripper confirm it either way?

[ballisticboy]

I looked at this very effect once based on the computed air pressure in the barrel of a springer at the moment of pellet release. I wrote a small time stepping program that computed the flow over the back of a pellet. There is a change in velocity but it is nominal, a fraction of 1ft/sec. It may be higher for a PCP but it will tend to vary depending on barrel length, pressure etc.. If the effect is large it will not be good for dispersion as it could affect pellet yaw effects.
I have not looked at small arms bullets but, unless the barrel was very short, I would be surprised if the increase was as large as stated since it would suggest a very inefficient charge being used with a very late all burnt position. I am not saying it is wrong, just surprising. It would also be bad for bullet dispersion as mentioned above.
The use of silencers or suppressors would increase the effect as the gas would still be constrained after leaving the barrel.

[angrybear]

I would think it's the loss of friction that allows/causes the projectile to continue to accelerate after leaving the barrel.

[Rich]

Air will always take the easiest path.

When the nose of the pellet emerges from the barrel, the air is still pushing the pellet forward. At the instant the back edge of the skirt leaves the crown of the muzzle, an annulus opens up behind the pellet. Initially this is tiny, but by the time the pellet has cleared the crown by a quarter of its diameter, the area of this annulus is the same as the area of the back of the pellet. The math is simple.

At this point the air can go either of two ways, follow the pellet and carry on pushing it, or dive sideways and avoid the obstruction. I'm not suggesting that the air has intelligence and makes an informed choice in this, by the way, but I believe the air will tend to avoid the pellet. I haven't given any significance to the momentum of the air, as its mass is so small.

Thus, if there is any additional acceleration once the skirt has left the barrel it will be very small indeed as the force necessary to bring about that acceleration (force = mass x acceleration) will only bear on the pellet for about 1 millimetre of travel, when the pellet is travelling at something like 240 metres per second, say four microseconds.

[ballisticboy]

Air will always take the easiest path.

When the nose of the pellet emerges from the barrel, the air is still pushing the pellet forward. At the instant the back edge of the skirt leaves the crown of the muzzle, an annulus opens up behind the pellet. Initially this is tiny, but by the time the pellet has cleared the crown by a quarter of its diameter, the area of this annulus is the same as the area of the back of the pellet. The math is simple.

At this point the air can go either of two ways, follow the pellet and carry on pushing it, or dive sideways and avoid the obstruction. I'm not suggesting that the air has intelligence and makes an informed choice in this, by the way, but I believe the air will tend to avoid the pellet. I haven't given any significance to the momentum of the air, as its mass is so small.

Thus, if there is any additional acceleration once the skirt has left the barrel it will be very small indeed as the force necessary to bring about that acceleration (force = mass x acceleration) will only bear on the pellet for about 1 millimetre of travel, when the pellet is travelling at something like 240 metres per second, say four microseconds.

The air will flow out of the barrel just as it does out of any other nozzle. The crown will direct some of the air at different angles but most of the air will continue to flow in the direction of the barrel like a slowly spreading jet. The important factor is the rate of slowing down of the jet as it mixes with the atmospheric air. The pellet will continue to accelerate while the air is travelling faster than the pellet. The length over which this situation exists will be small but more than 1mm. I cannot remember the exact distance as it is some years since I carried out the calculation. It is a standard intermediate ballistic situation and is important mainly for projectile dispersion effects. The force acting on the pellet will depend on the drag coefficient of the pellet going backwards and the relative speed between the pellet and the jet flow. The air from the jet will flow around the pellet the same as it would any other time the pellet is going backwards.
So the effect is marginal and the pellet is only in the flow for a short time. As I said before the time and magnitude of the effect will depend on a lot of things including the pellet velocity and the gas properties.

[RobF]

I would think it's the loss of friction that allows/causes the projectile to continue to accelerate after leaving the barrel.

Something only accelerates if a force is applied to it, not if a force is removed from it. Newton's first law.

If the friction of the barrel is removed then all that happens is it continues at that speed, it doesn't get faster because that requires another force to accelerate it.

The reason why people see slightly higher MV away from the muzzle is the index of refraction issues caused by the shock wave, which places the pellet visually in the wrong place to the chrono. A few inches away the shockwave is weakened enough that this doesn't happen to the same degree.

Rich is right, and air wants to expand in all directions. A lot of people confuse air flow with air pressure, and they aren't the same thing. You can see flow from reflex muzzles moving for seconds after a shot is fired, and it can travel quite some distance for some time, at least 2 feet. However the pressure shockwave is long gone and much shorter, it moving at the speed of sound.