Muzzle blast acceleration

[Inspector 71]

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

Love it.

[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 . . . . .

[ballisticboy]

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.

Once the pellet has left the barrel the air will flow out of the barrel the same irrespective of the pellet until it colides with the pellet base at which point it will flow around the base of the pellet producing an accelerating drag force.

Why is it assumed the airflow is faster than the pellet?

Have a look at this

25 secs in.

http://www.mirror.co.uk/news/world-n...er-man-9388648

I'm not sure why there's vapour before the shot, perhaps it's Co2 condensing on a pressure wave from inside the barrel, but it doesn't flow out faster than the pellet before or after the shot. You can see the vapour getting kicked by the shock wave before the pellet exits.

It also isn't clear that's the end of the barrel. Lots of 10m rifles have barrels shorter than their muzzles. Not familiar with the FWB in that respect. Walthers and some Anschutz's do.

The pressure in the barrel compared to the atmospheric pressure will decide the gas velocity from the barrel/nozzle with small modifications for the nozzle efficiency. There is a fairly simple relationship between the gas pressure and the resulting gas velocity. Just standard gas dynamics. The gas pressure in a springer is sufficiently high to produce high gas nozzle speeds let alone from a PCP. So I am not making any assumptions, it is just what the numbers tell us must be happening.
Gerald Cardew photographed airgun darts with the stabilising tufts being blown forward by the gas from the barrel many years ago. Pellets are no different, you just cannot see the effects as easily, you would need to use a Schleiren system to see it properly as a shadow graph would not be sufficient.

[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.

[RobF]

Once the pellet has left the barrel the air will flow out of the barrel the same irrespective of the pellet until it colides with the pellet base at which point it will flow around the base of the pellet producing an accelerating drag force.

The pressure in the barrel compared to the atmospheric pressure will decide the gas velocity from the barrel/nozzle with small modifications for the nozzle efficiency. There is a fairly simple relationship between the gas pressure and the resulting gas velocity. Just standard gas dynamics. The gas pressure in a springer is sufficiently high to produce high gas nozzle speeds let alone from a PCP. So I am not making any assumptions, it is just what the numbers tell us must be happening.
Gerald Cardew photographed airgun darts with the stabilising tufts being blown forward by the gas from the barrel many years ago. Pellets are no different, you just cannot see the effects as easily, you would need to use a Schleiren system to see it properly as a shadow graph would not be sufficient.

They are a little different, a dart is a slow projectile that doesn't seal at all in a barrel, where as we know a pellet does to a somewhat better degree and travels a lot faster.

How fast was the dart going?

We can work out the pressure in the barrel. Just done that above.

Schlieren would show it, but all i've seen of that in subsonic hasn't shown any flow past the projectiles. The catch is that I haven't seen the proximity to the muzzle so it's difficult to tell what is happening, except that you can see under some circumstances it doesn't pass the projectile (whatever they may be).

[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]

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.

[RobF]

Interesting. Thanks.

[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.

[bucketboy]

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.

The whole point of a moderator is to reduce the speed of the supersonic air as it leaves the barrel, every mod I have made has had a measurable but sight reduction in muzzle speed.

There is a volume of air between each baffle, as the pellet enter that space it slightly compresses the air trapped inside before the pellet moves through the next baffle into the next space where is slightly compresses the air trapped inside that and so on until the pellet exits the end of the moderator. Every chamber the pellet enters it looses a little bit of energy compressing the air inside, more baffles, more chambers, less energy.

Multi baffle mods sap more power than the same length mod with fewer baffles, longer mods sap more power than short mods.

If you don't believe me, build a mod say 300 mm long, 32mm dia, fit baffles every 25mm take a few shots over the chrono, do it again with baffles at 50mm again at 100mm then with no baffle, I would like to bet there is a measurable difference

[Archer50]

Thanks Guys. I do seem to have a knack for asking what I thought was a simple question and nearly starting a civil war (see the thread when I asked about 'taking the wind'!)

I simple terms, the answer to whether an airgun pellet keeps accelerating after it leaves the barrel seems to be 'possibly, but the effect on final velocity is not significant in any practical way'.

Cheers

Alan

[Rich]

I did some quick sums on the figures ballisticboy suggested and the MV increased from 240 metres per second to just under 240.1 metres per second.

More detailed sums might prove me wrong but I don't have the time or the enthusiasm.

[ballisticboy]

I did some quick sums on the figures ballisticboy suggested and the MV increased from 240 metres per second to just under 240.1 metres per second.

More detailed sums might prove me wrong but I don't have the time or the enthusiasm.

Finally managed to dig out the simulation I did 4 years ago. I used a springer as the basis and the numbers were much as I remembered with a velocity increase on the pellet of 0.2 ft/sec due to the gas jet. Where my memory was wrong was that the numbers suggested the pellet was inside the jet from the muzzle for around 2 inches, not the less than one inch I thought. The jet velocity slowed down very quickly with most of the effect very close to the muzzle as you would expect.

Basically I would say forget it, the normal variability of the pellet muzzle velocity will be much greater than this effect which is why I did not bother to publish the numbers when I carried out the simulation.

[RobF]

That means that although it happens any noticeable recording of mv increase away from the muzzle isn't happening due jetting. As you say you can't get a gun to chrono accurately enough to notice a 0.2 fps increase.