Thats cool.
I built one previously, it was more an air cannon, I used PVC pipe and a valve.
Essiently to make the gun, and then once the frame was completed, I used a cap to seal out the end and put a little valve tube thing(sorry don't know what its called, like a bike tire inflator) and then would pump air from the air machine/inflator we had, it had great psi, and then once the valve was released, dang, the stuff shot would fly. I still remember breaking the neighbors window by shooting a frozen tomato, lol.