SpudFiles

I had this idea for a self filling piston cannon, I want to know if this idea should work, or if it has been done before, and how many shots above 60 psi (if I pressurize it to 120) I can get on one tank.

please use accurate terms this is not "a self filling piston cannon" this is a manual filled piston valve. I've seen plenty of variation of this

From my experience the pressure drop on non regulated air supplies is relitivly huge, I wouldn't have much hope for a reserve made of 2" pvc
if you want to calculate the pressure drop for your own particular design use the equation
P1*V1=P2*V2 or (P1*V1)/V2=P2
P1=the initial pressure in the air reserve (what your referring to as the main chamber)
V1=volume of the air reserve (what your referring to as the main chamber)
V2=the volume of both chambers combined
P2=the pressure of both chamber when the fill valve is opened to

btw you have a ridiculous amount of dead pilot volume and too much space for the piston to move, a sprinkler valve to pilot a 3/4" piston valve is over kill, just go with a back pack air supply connected with vinyl tubing

How do you think I whould reduce the pilot volume then? I drew it as small as possible. The coupling and bushing could be changed to a reducer, but thats about it... or at least all I could think of. And as for the pilot valve, I have a SV I'm not using, so I figured I could use it.