Attached files contain components for 3d printed cannons as an example of what can be done. These have worked well for me. Your mileage may vary.
Note that piston length and base diameter optimization are important.
an O-ring is used to seal the back of the breech, I will get dimensions and sizes of piston and O rings when I find the bags.
Barrels are printed undersized on bore and reamed with a 0.172 diameter reamer