D_Hall wrote:You don't need much power. I'd go with the 3/4" QEV and a metal chamber. All-metal = more robust. The last thing you want is the gun to come apart due to mishandling and the powers that be to blame it on "poor worksmanship" and/or "bad design."
That's what I would do if I were in your position, if you use PVC there are a lot of extra hazards that aren't really necessary (especially since you're using regulated CO2). I would say go with the 3/4" QEV and a galvanized steel chamber.
I have a 3/4" QEV on my tennis ball cannon, and it has plenty of power, enough to launch a tennis ball 150 yards.
EDIT: If you already ordered that 2" sprinkler valve, you can make yourself a cannon with it.
It's hard to soar with eagles when you're working with turkeys.