Fuel Air Gun

When a conventional air gun fires, it's spring loaded piston compresses the air in front of it to high pressures, and this air in turn propells the bullet out the barrel.

The compression of the air produces some very high pressures (for a short period of time).

I propose a gun very similar to an air gun, except that before the piston goes forwards, combustible fuel is injected into the air before it's compressed. The heat of compression would ignite this fuel, and the heat of combustion would increase the power of the bullet.

Almost any kind of liquid or gaseous fuel could be made work -- alcohol, gasoline, butane, etc.. With the right injection timing, powdered fuels (charcoal or coal dust, or even flour, for example) might also work, provided they don't produce too much residue after combustion.