SpudFiles

In an auto / semi-auto firearm, a hole is placed in the barrel to allow some of the gases from the round to travel to a piston that resets the bolt and readies the next round. Here is a link that explains it better.

My question is, is there a formula or something to calculate the pressure on the piston that is reset by these gases?

Edit: Also, which would better for pneumatic applications, long (ported near muzzle, travel > diameter) or short stroke (ported near center or end, travel < diameter)?

GGDT will tell you what pressure you have in the barrel at a given unit of projectile travel, you can add the volume of the piston tubing to get a value.

Remember however that these sort of firearms typically generate 30-50,000 psi - well below spudgun territory - so I doubt this system would be viable for our sort of volumes and working pressures.

The air powered FX Monsoon (which I'm still waiting for ) apparently uses a similar system, however it has well over 3,000 psi in the tank.

I think the best idea (as used by many relatively low pressure submachineguns) is direct blowback, that takes advantage of the fact that in the breech, the pressure is at its highest.

Thanks.

Another question: How could the blowback system by utilized with pneumatics?

I used the system with a measure of success in my cartridge fed pneumatic that used loose tolerances (because it had a separate valve) and was therefore low on friction and could cycle - albeit too rapidly to reload another cartridge, so I had to add a bolt-stop to give semi-auto fire.

My current project would be blowback in an ideal world but since the bolt is effectively the valve, I had to make it very timght to minise leaks and this means that the air pressure alone is not enough to cycle the bolt.

What I could have done to boost the force on the bolt is something like the attached diagram, with a much larger diameter piston attached to the bolt that would increase the available area for the air to act on.

JSR I suggested you in your post that your design would work better with greater caliber - but yeah this is exatly what you are looking for

hope it's gonna work

i might build that^^^^^^

it looks like it would actually work and not be too much of a hassle. expect more from me....... lol.

POLAND_SPUD wrote:JSR I suggested you in your post that your design would work better with greater caliber - but yeah this is exatly what you are looking for

hope it's gonna work

It's a hassle with my construction method to make coaxial pistons that work, but I guess if I managed with blow-forward valve I could make this work too.

I meant that greater caliber = greater bolt surface = more force acting on the bolt....

anyway it would be nice to see a fully auto 12.7 mm spudgun...

JSR are you going to use the air on the other side of the bolt/piston to slow down the bolt ? (as a kind of an air spring ? ) i suppose that some flow control valves could be used to regulate the amount of air let out - possibly it could be used to choose RPM or something...

I meant that greater caliber = greater bolt surface = more force acting on the bolt....

True, but that also means larger, heavier bolt, with more travel, that requires a stronger spring - in reality, that would just scale up the problem.

JSR are you going to use the air on the other side of the bolt/piston to slow down the bolt ? (as a kind of an air spring ? ) i suppose that some flow control valves could be used to regulate the amount of air let out - possibly it could be used to choose RPM or something..

It's a good idea on paper but to use it as an air spring means an airtight piston, and that implies more friction which I already have enough of... it's a tough equation to balance