know i can get more power out of a pneumatic but the problem is that i don't have a compressor and if i were to buy one portability will be an issue. And I don't think pumping up a dual chambered air cannon with a bike pump will be too efficient...
If you don't have a compressor, buying one will be expansive and actually the cycle time will be just as long if not more long than a propane one. The bigger the tank, the smaller your compressor, the longer it will take you to shoot off a couple shoots.
My final word, go for a combustion. I am tellin' ya. Go for combustion. I have both and I enjoy my combustion for big projectiles and my air riffle for small projectiles. Here's a link to EASILY calculate your ratios and meter length, pressure etc. I found it easy to use and that using it made the construction of my combustion canon A LOT EASIER.
You should know that the fuel mixture and atmospheric pressure has to stay as it is.