
It's 6" x 60" -- I found it among some spare pipe somewhere I once worked last year, with epic plans in mind. Meanwhile, I got married and a new job last summer and pretty much forgot about it.
I once had a pneumatic I made with a 6" x 42" chamber and a piston valve I made from a 3" endcap. It was kind of ghetto, but it worked reasonably well with a 2" x 20' barrel. But I discovered chronic leakage around the chamber endcap and coupling (due to my own mistake)... I scrapped it in fear of my safety (After all, 6" PVC sch40 is only rated to 180psi).
So, I've been leaning towards a combustion. I have a pretty much limitless supply of propane gas, making that part cheap. I've also wanted to experiment with a new breech design I've been batting around inside my head recently, which would be unsuitable with a pneumatic (unless I made a chamber-sealing piston valve).
I also have a few 10' lengths of 4" and 3" pipe here, I will likely use 3" for whichever I go for.
So whatcha think, go ahead with the combustion or make a pneumatic again? This will definitely be the largest cannon I've made to date (and the first I've felt was worth posting about since I started posting here again!)
All those fittings are dry-fitted right now. 6" endcap and coupler, 6" socket-weld to 4" female threaded bushing, 4" threaded on one side nipple, and 4" tee.