Kind of off-topic, but just FYI, I shoot 12 gauge shotguns a lot. My regular combustion gun kicks more than a 12 gauge if you have a heavy projectile in it. Launching a caulk tube full of concrete kicks about like a 10 gauge. I've never shot an 8... what's that like?
I've never built a hybrid. I intend to build one some day, though. In my opinion, shoulder-firing a PVC hybrid cannon is borderline suicidal. I'd only shoulder-fire a hybrid if it was made of steel and the welder was very skilled. The problem with that is, it'd weigh 45lb. Just build a nice stand for it and a 50-foot remote detonator
For those that don’t know what a hybrid is, and are too lazy to search the forum a bit, a hybrid uses both principals of combustion and air pressure. In a normal combustion gun, you use a mix of roughly 4% propane to 96% air. In a hybrid cannon, you inject twice as much propane as normal (for a 2x mix), and then compensate for the lack of oxygen by pressurizing the chamber with air, to bring the balance back to 4/96 (but at a higher pressure). A 2x mix is twice the "normal" amount of propane and air, and so on for 3x and 4x mixes. The compressed fuel/air mixture is kept inside the cannon by a burst disk, which ruptures upon detonating the fuel.
Obviously, pressurizing an explosive gas with air inside a piece of plastic pipe and then detonating it is a reasonably insane thing to do. Exactly why so many of us are intrigued by it. But seriously, don't fire one of those monsters from your shoulder. Normally I wouldn't care; if you blow yourself up you blow yourself up, but the way I see it, if enough people get hurt with spudguns, they'll be made illegal. So in the interest of keeping potato cannons from getting legislated into oblivion, don't do it. Thanks.
Peace,
Pete Zaria.