Several years ago, the Tippmann paintball gun company came out with a Propane powered gun that ran off of small non-refillable propane canisters (the type used for small camping stoves). They claimed the gun got 50,000 shots from one 16oz can. When the gun was pumped, it loaded 1 paintball and charged a combustion chamber with a mixture of air and propane. Pulling the trigger created a spark to ignite the mixture and the ball went hurtling down range at 300 f.p.s. The gun never really caught on, since it's a pump in a field dominated by 12+ ball-per-second semi-autos (I wanted one!)
It would be very interesting to apply this system to an air rifle. I'm sure the BATF would be knocking on my door if I ever made one though.