It's funny this thread has moved to making this full auto. I don't have any sketches yet, but I have been designing one. Here is how it would be built;
1 The piston has no rod, but does have a small EQ port. The rear of the cannon is sealed and only contains a bumper.
2 The chamber has a pop off that vents into the barrel directly between the piston and projectile.
3 The barrel has a small post to prevent launching the piston.
This is how it would work;
1 Piston is closed and the chamber pressure builds until;
2 The pop off pops open and provides a blast of air between the projectile and piston which;
3 launches the projectile and piston in opposite directions opening the main charge into the barrel. This rise in pressure in the barrel then;
4 vents some into the pilot rising the pilot pressure beyond the pressure of just displacement compression. As the projectile exits and the chamber pressure drops;
5 The pilot area pressure closes the piston sealing the chamber and the remaining pilot area pressure finishes venting back out the EQ hole in the piston.
Process repeats full auto fashion. Think it has a chance of working?