Here is a close up view of how I use a 3" tee and a cleanout to breach load my cannon. The cleanout cap has a chunk of 3/4 sch 40 pvc glued to it to keep the projectile ahead of the air flow from the valve. It works really well.
Here you can see the whole thing with my cannon stand (well not really cool, but it works). The airline is attached to a ballvalve to allow filling, so long as the compressor can keep up I can fire quite a few shots in a short peroid of time, just fire, close the firing valve, open the breach, load, close the breach, open the fill valve, watch the guage, close the fill valve, FIRE. It sounds like alot, but with practice it can be done supprisingly fast. Probably about 8-10, 50 psi shots in a minute.
Here is just a pic up the barrel, nothing special, but you can see how I secure my barrel, the red straps are ratching tie down straps and I have marked the barrel to the barrel support and the barrel support to the tank, and after about 50 shots with a 1.5 lb bottle full of water the barrel didn't move a bit. I still am working on building a cannon mount with adjustable elevation, by means of turning a crank. I will calibrate the crank to minutes of angle, or maybe mills, so it should be fairly accurate.