Originally Posted by Duckman
hpa can work as long as you regulate the pressure to those similar to what you'd use with co2, say 400-600psi-ish.
Nope. HPA won't work! HPA is air which remains in gas form when compressed. CO2 when compressed is in liquid form. Liquid CO2, much like Propane, is transferred to your grenade. (thus having the tank inverted when filling and also why Carl won't let us play with his toys!!)