To put recoil, and a few other things in perspective: Even with a 12' long 2" SCH 80 barrel (which would be terribly heavy and unwieldy), this thing would barely be topping Mach 1 with a 500g load, assuming that it could even overcome trans-sonic flow effects (which is quite possible considering the relatively high pressure). This would give a muzzle energy of 26k ft/lbs, by a very liberal approximation. The Mythbusters' "Old Moses" 3.5" bore cannon fired 8 pounds at Mach 1.5, and just incase you didn't notice, the operators weren't horribly killed by it. If recoil becomes a problem, just add weights to the cannon, or beef up the supports.
My ME20H-41 does 10k ft/lbs and weighs a tiny 50 pounds, and it only kicks back 3 feet or so without any brace whatsoever.
As for a barrel: Suprisingly enough, I made a barrel that survived several shock loads of up to 2000psi by sleeving 1.5" ABS pipe inside 3" ABS pipe, and filling the space with structural fiberglass. If you want to go the non-ghetto way, and are prepared to sell the soul of your firstborn to UPS, you could get a SCH 80 steel pipe shipped to you from McMaster-Carr.
you would get a metal stand with car shocks and and electronic egnition system i may be crazy but i am not dumb
Wait, is this going to be a pneumatic, or a hybrid? Because I have to say, if you plan on trying a 3kpsi pre-ignition hybrid in a SCUBA tank:
1. Propane/air liquifies at ~190x, well below 3kpsi.
2. If you do it, by using hydrogen or something, the tank will be ripped to shreds, and so will all of your expensive fittings, and anything within 10 feet of this thing when it goes off.