Muzzle blast acceleration

[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.

[Rich]

quote

The air will flow out of the barrel just as it does out of any other nozzle.

unquote

I'd agree with that if there wasn't a pellet in the way.

[max headroom]

all to do with the twist in the barrel. the more rifling the faster the bullet will fly. it screws trough the air.

[Shed tuner]

all to do with the twist in the barrel. the more rifling the faster the bullet will fly. it screws trough the air.

you're a week early

[ballisticboy]

all to do with the twist in the barrel. the more rifling the faster the bullet will fly. it screws trough the air.

Don't joke, it has been seriously proposed to put a propellor on the back of a shell to make it go further.

[Rich]

Don't joke, it has been seriously proposed to put a propellor on the back of a shell to make it go further.

That's clever; they could hitch a wind turbine to a Boeing 777 and make it more fuel efficient, then.

[air-tech]

No surprise really, but I vote ballisticboy.

I think another factor will be the pressure build up ahead of the pellet.

And how much pressure there actually still is in the barrel when the pellet is at the muzzle.

However - with a moderator the effect of blast is measurable. Again no surprise. And again the factor above is critical. A small but measurable and consistent increase in MV was noted on an FAC PCP when a moderator was fitted.

Possibly with a springer there may be none at all - at best the blast maybe just reduces the drag, thus reducing the deceleration, rather than causing acceleration. This would seem to be the case not far beyond the point where any extra barrel length would cease to accelerate the pellet. It *will* be slightly beyond - as not having a barrel from that point would mean that there is no friction, thus the pressure there is still sufficient to accelerate a bit more, as some of it is currently just overcoming friction.

[Rich]

I've measured tiny enhancements with silencers on PCPs too. Maybe 1 to 2 fps.

Springers use something like a third as much air as a PCP - all else being equal- so I would agree with you that any effect with a springer would be even smaller.

[max headroom]

No surprise really, but I vote ballisticboy.

I think another factor will be the pressure build up ahead of the pellet.

And how much pressure there actually still is in the barrel when the pellet is at the muzzle.

However - with a moderator the effect of blast is measurable. Again no surprise. And again the factor above is critical. A small but measurable and consistent increase in MV was noted on an FAC PCP when a moderator was fitted.

Possibly with a springer there may be none at all - at best the blast maybe just reduces the drag, thus reducing the deceleration, rather than causing acceleration. This would seem to be the case not far beyond the point where any extra barrel length would cease to accelerate the pellet. It *will* be slightly beyond - as not having a barrel from that point would mean that there is no friction, thus the pressure there is still sufficient to accelerate a bit more, as some of it is currently just overcoming friction.

think about the double spring action. the spring goes boing and sends the pellet off. then the spring comes back and sucks the pellet back. not all the way back.

[clunge]

A 3% increase in velocity represents quite a substantial amount of energy imparted to the projectile.

Sounds very unlikely to me. So I'm with our ballistic superhero too.

[robinghewitt]

Not forgetting that the pellet is travelling at 4-500 mph. I seem to remember the poof celebre, James May, explaining why the Buggati Vehron, however it spells itself, couldn't go any faster without vastly more power because the air was the consistency of fruit cake at it's top speed. A top speed which was considerably slower than an airgun pellet

[Rich]

think about the double spring action. the spring goes boing and sends the pellet off. then the spring comes back and sucks the pellet back. not all the way back.

note to TD: must get Jim T on to this one, to analyse which element of the springer firing cycle exhibits a partial vacuum . . . . .

[RobF]

No surprise really, but I vote ballisticboy.

I think another factor will be the pressure build up ahead of the pellet.

And how much pressure there actually still is in the barrel when the pellet is at the muzzle.

However - with a moderator the effect of blast is measurable. Again no surprise. And again the factor above is critical. A small but measurable and consistent increase in MV was noted on an FAC PCP when a moderator was fitted.

Possibly with a springer there may be none at all - at best the blast maybe just reduces the drag, thus reducing the deceleration, rather than causing acceleration. This would seem to be the case not far beyond the point where any extra barrel length would cease to accelerate the pellet. It *will* be slightly beyond - as not having a barrel from that point would mean that there is no friction, thus the pressure there is still sufficient to accelerate a bit more, as some of it is currently just overcoming friction.

The apparent increase in MV with silencers and strippers is because the shockwave that displaces the image of the pellet to the chrono isn't present, it's been blocked.

The pressure at the muzzle can be calculated, you know the pressure and volume used per shot on a pcp, and you can measure volume between the valve and the muzzle. We've been strapping digital gauges to bottles and regs for a while when we've been playing with valving. They're good to .00 bar so we have a fairly good idea even on small volumes.

As a rough estimate my 660mm LW has about 10 cc after the valve to muzzle.
The Walther got a 150cc bottle on it say at 200 bar which drops about 0.8 of a bar each shot (need to do a more accurate test on this).


So 0.8bar x 150cc = 120 cc at atmospheric.

After the valve you've got about 10cc, so that makes the pressure at the barrel about 12 bar, assuming no leaks. It then expands to 120cc, which is about 1/3 of a can of coke.


That's if my maths is good this afternoon and obviously smaller barrels, different guns and valving can change that. I'm not convinced that being exposed to 16 bar for a split second is going to make that much difference considering that away from the muzzle that pellet's backside is going to be tiny just an inch away compared to the sphere of air (which is easier to push away) in comparison that's expanding.

[ballisticboy]

The apparent increase in MV with silencers and strippers is because the shockwave that displaces the image of the pellet to the chrono isn't present, it's been blocked.

The pressure at the muzzle can be calculated, you know the pressure and volume used per shot on a pcp, and you can measure volume between the valve and the muzzle. We've been strapping digital gauges to bottles and regs for a while when we've been playing with valving. They're good to .00 bar so we have a fairly good idea even on small volumes.

As a rough estimate my 660mm LW has about 10 cc after the valve to muzzle.
The Walther got a 150cc bottle on it say at 200 bar which drops about 0.8 of a bar each shot (need to do a more accurate test on this).


So 0.8bar x 150cc = 120 cc at atmospheric.

After the valve you've got about 10cc, so that makes the pressure at the barrel about 12 bar, assuming no leaks. It then expands to 120cc, which is about 1/3 of a can of coke.


That's if my maths is good this afternoon and obviously smaller barrels, different guns and valving can change that. I'm not convinced that being exposed to 16 bar for a split second is going to make that much difference considering that away from the muzzle that pellet's backside is going to be tiny just an inch away compared to the sphere of air (which is easier to push away) in comparison that's expanding.

A pressure of 12 bar being released into the atmosphere is enough to give an initial gas velocity of up to 1350m/s from a proper nozzle, much less from the end of a barrel but plenty for very high gas speeds. While the shock will travel out in a sphere the gas will not as it has all its momentum predominantly in one direction which in this case is up the back of the pellet. The effect is all over well before the pellet has travelled one inch but remember, 12 bar is enough to give a force on the pellet base of 19 newtons which will produce an acceleration of over 3850m/s/s. Now this will only give a very small speed increase because of the time but it will give some.