DYI wrote:For a two stage gun to be of any great utility, you're going to need a burst disk separating the pump tube from the barrel.
Remember lgg's shoot at hundreds of kilometers a second he is going for 1125+ fps.
Not really, light gas guns usually tend to peak under 10 km/s after which human propellants become insufficient to drive the piston hard enough.
It's the momentum that creates the pressure spike (at least, if you want the pressure to spike higher then the initial pressure source.)
In conventional light gas guns the pressure of the working fluid is not the limiting factor generally. It's the speed of sound in the working fluid. A heavier and slower piston will LOWER the shock heating caused by the motion of the piston, which is the reason for having the piston.
However in a comparatively extremely low pressure scenario it is up for debate as to whether slightly higher pressures are worth the drop in temperature. A heavier piston is also more difficult to bring to a stop without damage to equipment. Finally, materials such as HDPE are used in conventional guns as plastics are fairly cheap and easy to have turned to the correct size. Important seen as these guns aren't known for piston lifespan.
It's also worth noting that a light gas gun is hardly the most efficient machine ever. You have substantial losses through the piston, and then again through the new working fluid and then again through the projectile. The only purpose for constructing a gun like this is pure muzzle velocity.
which i guess goes back to square one when you sai that any attempt that is kept under 1 km/s is utterly pointless.
Not really. There is more than one reason for building a gun whether it be scientific research, for the experience, a personal challenge, building to criteria, improvisation..the list goes on.