Welcome, glad to have you aboard. If you build one out of PVC, be sure to use pressure rated pipe and no DWV fittings. Wait the recommended time for the solvent weld to set. Keep the pressure under 150 psi and dont use it in cold weather, and you should be safe. PVC fragments when it burst, kinda like a plastic grenade. The barrel length kinda depends on the diameter, but I'd say a barrel 24" to 36" should work well in 1/2" to 1" diameter barrels.
