Here is an air cannon I made with on old air compressor tank and some other junk I had lying around. The only cost to me was the ball valve that cost about $50.
I may need to move the pressure gauge to another available port that is not on the fill stem, I'm not sure if I am getting a true read.
The entire gun is made from steel and brass except for the barrel that is after the ball valve is PVC to reduce some weight.
I am also still working on an elevator to move the muzzle up and down for more accurate shots.I thought of using a screw/scissor jack but I think It may be too bulky and slow. I am open to suggestions if anyone wants to contribute....