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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.
[Custard]'s ether injection piston gun. This idea is baked (if obscure). [spidermother, May 02 2009]
||Ask any squaddie about lighter fuel powered onion guns.
So very very baked.
easily available propellant & cartridgeless: wonder how many "rounds" you could get out of a propane bottle.
||I'll do a google sometime when I'm not at work, but apparently there was a piston gun available in the 60's or whenever, that had ether injection (ether has a low ignition point, or some other useful property). Apparently there were accuracy issues with varying muzzle velocity - possibly newer technology could reduce this, but I'd wager there isn't a way to get the power spot on repeatable.
||The process is called dieseling, by the way. Info is hard to find on the web, but apparently over-oiling your piston gun can have a similar effect (rather obviously).
||A few years ago I was toying with the idea of making a squeeze-bore gun with this arrangement. Unfortunately piston guns will always have high recoil and mass, so I'm not sure how popular it would be in comparison to increasingly cheap, and disturbingly powerful compressed gas guns available now.