Chamber size (nominal):4in x 65in
Barrel size (nominal):2in x 96in
Piston: 4in PVC rod
Sealing face: Old mouse pad
Main firing pressure: 30psi (due to space limatations)
Max firing pressure: ~120psi
Main ammo: Cast concrete slugs, Golfballs
Range at 30 psi: ~700-800 ft (Golfball)
Here is the completed cannon:

Here is a closeup of the back:

Here is the barrel with the front glued on:

Here are all the parts laid out:

The back has the fill valve, a safety pop off valve, and a 1in ball valve for venting. Currently fired at around 30psi due to space limitations.