Automatic Dieseling Air Gun

First, I should mention that this idea is more of a firearm than an airgun.

Imagine you've got the following components, from back to front: An air intake. A carburetor. A checkvalve that lets air move from the carberator to the spring chamber, but not backwards. An airtight chamber with the spring in it. A piston with a checkvalve in it. The check valve lets air move through the piston to the combustion chamber. The combustion chamber. The bullet. The gun barrel.

When the trigger is squeezed, the spring pushes the piston forwards, simultaneously drawing a fresh fuel/air mixture from the carburetor into the spring chamber, and compressing the mix in the combustion chamber.

When the pressure and temperature in the combustion chamber get high enough, the fuel/air mixture combust, creating a large increase in temperature and pressure.

The hot gasses simultaneously press the bullet forward and the piston backward. As the piston moves back, the fuel air mix behind it compress. After the pressure in the spring chamber matches the pressure in the compression chamber, the additional backwards motion of the piston will force the fuel air mixture through the piston's check valve into the combustion chamber.

Once the piston is all the way back, it would either be caught by a latch (if the gun is in semiautomatic mode) or be allowed spring forward (the gun is in full automatic mode).

Meanwhile, a small portion of the high pressure exhaust gas is used to load a new bullet. The new bullet prevents the air/fuel mix in the combustion chamber from dissipating out the barrel.

There are a couple of obvious benefits over a regular gun: Firstly, a wide variety of fuels could be used, probably even regular diesel fuel. Secondly, since the bullets don't need shells, the weight of ammunition could be vastly reduced. Thirdly, it might be possible to make the gun's power adjustable by varying the air/fuel ratio.