The principle behind what you describe is called 'deiseling'
a small amout of fuel like oil or similar is injected through the transfer port of a spring-piston air rifle. as the spring propels the piston forward the air in front is copressed, this causes heat & ignites the fuel causing an explosion & much higher pressure than the rifle was intended to produce. This will result in an high increase in velocity but there are problems:
1, The air cylinder that the spring & piston opperate in were never designed to handle that explosion and so can bulge out, crack or rupture rendering the gun useless.
2, Most piston heads are made of a composite or a leather washer. these just burn up or melt rendering the gun useless.
3, Velocities are very inconsistent and accuracy goes out of the window.
4, Most everyday airgun pellets are not designed to work under these pressure & deform so again don't expect to hit what you aim at. There are some heavy airgun pellets nowadays that are made to witstand higher pressure but these are intended for modern pre charged pneumatics. Some people have used .22LR bullet heads.
5, From a legal aspect, fuel or propellant is being used instead of air so you now have a firearm, not an airgun.
Most spring piston air rifles do slightly diesel to a very small extent & thats fine but to induce it on purpose with the intension of higher velocity will destroy the gun in no time at all.
WARNING: Please do not try this, it is very dangerous!
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