Sure they are. Its the only source of energy in the system.
True, to a point. But you misunderstand me, I wasn't clear. From Wikipedia:
"The limiting factor on the speed of an airgun, firearm, or light-gas gun is the speed of sound in the working fluid—the air, burning gunpowder, or a light gas. This is essentially because the projectile is accelerated by the pressure difference between its ends, and such a pressure wave cannot propagate any faster than the speed of sound in the medium. The speed of sound in helium is about three times that in air, and in hydrogen 3.8 times that in air."
That is what I was getting at. And I thought hydrogen was six times, but if it is only 3.8 vs. 3 for helium, it would be much safer to use helium. I want a light gas gun, not a combustion light gas gun (although that would be MUCH cooler!). The combustion light gas gun has already been weaponized, the light gas guns I know of are scientific devices. In combustion light gas guns the gas is set off with a spark plug getting even more velocity (and pressure).
I don't want to use a cylinder that tough, or a piston that precise, and I don't want a design that complicated. The whole idea is to keep the single shot simple. So simple it will have a just a disc that will rupture at the right pressure thus allowing the light gas to push the bullet down the barrel blank. The barrel doesn't even need a chamber, just enough material removed to allow a bullet to seat up to the rifling. I'm thinking about a .223 since it would probably be the safest and fastest combination I could do. Certainly couldn't use run of the mill bullets or twist rates.
As a safety to keep the piston/barrel assembly from blowing out, I figured I'd vent the chamber just past the point where the piston compressed the gas enough to blow out the valve. Think porting, several holes drilled in line with the cylinder.
Overall it should be fairly safe, and of course I'd be safe enough to remote fire it. The prototype would probably be hard on parts, particularly the piston, the big problem is how to stop it without it becoming damaged or damaging the barrel assy. But that is how you learn.
Eventually I'll get the equipment I need, the mill and the lathe, and although I don't need it for this one project, my mfg. license (I want to build rifles and AOW's, mostly umbrella and cane rifles). I'll have to shelve the idea for now. I just thought some constructive discussion could come out of here, perhaps some ideas, but I guess not. Typical internet shallowness... Please close this, delete the whole thing, whatever. Not going where I wanted it to go.