For combustion guns the pressure of the expanding/exploding gasses must be kept in mind. PVC pipe depending on its diameter is only designed to contain a certain working pressure load. Going past this point (the maximum pressure rating) is not only dangerous, but stupid as well. Doing so not only stresses the pipe, fittings, & seams, but also increases the chances that it WILL fail at some point. Keep this in mind when you decide to start tweaking your combustion gun to increase performance.
Things are a little bit different when it comes to pneumatic guns. The smart thing to do is to have a [pressure guage] on you gun so you can montior the pressure it's running at, making sure not to exceed the max rating for the pipe. (If you have multiple sizes of pipe, go by the pressure rating for largest piece of pipe, or the weakest point.) A properly adjusted [blow by valve] is also very useful. With a blow by valve, you don't have to keep a super close eye on the pressure; Only to air it up, and fire. If the (pre-set) pressure point is reached, it will automatically vent air from the pressure chamber to keep the pressure below the maximum level.
Combustion guns require a fuel, as well as fresh air to fire a round. That said it means that you must have a flammable/explosive fuel for each shot, as well as vent the chamber with fresh air before the next firing. However, combustion guns tend to be a tad bit more portable than the pneumatic ones. Reason being this, pneumatic guns need some sort of compressor to charge the chamber. Albeit you don't have to have an air compressor, they make life so much easier, thus avoiding pumping them up by hand. However there are also battery powered compressors, as well as small gasoline powered ones that are fairly portable. So depending on the amount of money that you'd like to invest, you'll limit yourself to what you can do with the hobby of spudding.
On to the Combustion Guns.