The simplest form is a short piece of pipe with a [clean-out cap] on one end & a [reducer] (bell/bushing) on the other end with a barrel. A [Coleman flint sparker] is used to ignite the fuel. These are cheap to buy & simple to use, but sometimes are cranky when sprayed with too much hair spray, or due to atmospheric conditions (read Moisture). If you want to go high tech you can opt to use some form of electronic ignition. There are several types: [piezo electric], and [stun gun], both are used to create a spark to ingite the fuel-air mixture. Peizo electric are the electric ignitors that are in grills, Aim 'n Flames, & lighters that don't have flints. Stun Guns, true to their nature makee large sparks & are pretty dependable, but can shock the shit out of you if you're not careful. The advantage of electric over flint is that you don't have to replace flints & generally a cleaner & hotter spark for better ignition & a more complete burn. (Sounds just like a Split-Fire spark plug ad eh?) Which ever you choose, flint sparker or electronic, both require that you drill holes somewhere in your gun.
That's about it for stuff to consider when pondering on combustion; You have the issue of flammable/explosive fuels, & buying them, the noise that they create, the slower reload time, & what type of ignition you would use.
Up Next are the Pneumatic Guns.