Propane
The most common fuel of high tech builders. Often metered into the chamber at 4.2% by volume, because you lose power if your fuel/air mix is wrong. However, it can be manually added trough (for example) a non-lit propane torch, as you would with an aerosol. The torch head mixes the propane with air, filling the chamber with a finished mixture, so overfueling using this method isn't a problem. This is especially convenient in pill bottle guns, where traditional metering isn't practical.
Propane is often the primary propellent in aerosols.