This is a High powered pneumatic gun. I've made two barrels for it so far (the one you see wich fits nerf darts) and a 1/2" PVC barrel which fits marbles. The range on this thing is incredible. With nerf darts and running 120psi I can get about 50meters, with marbles at 120psi I can get about 120meters.
I've built it to handle about 200psi but havent got a compressor to fill it up that high.
I know you wont be able to make something like this unless you can TIG weld but I thought I would share it anyway.