This is one of my micro spudguns. It has a 1/2" (pipe size, not actually that large) schedule 40 PVC chamber rated to several hundred psi. It has a fan to mix the fuel-air mixture before deflagration, and a 3 gap sparkstrip to create 6 flame fronts rather than two. It also ignites the fuel-air mixture more reliably.
Fuel (butane or propane) is metered from a small syringe and directly injected into the chamber. The barrel threads are 1/4" tapered NPT. There is a schrader (automotive valve) added for chamber clearing and hybridding. This cannon can also use a burst disk.
By the way, for those who do not know, an advanced combustion (spudgun) usually has: metered fuel injection, chamber fan, spark strip, breech loading (loads from back of barrel, not the front) and whatever other features. Not your normal spray 'n pray.