[BuffaloJack]

This is excellent advice for firearms, but don’t do this with air rifles. Today’s air rifles use pressure high enough to initiate dieseling. An air rifle designed to shoot a pellet at 1200 fps may launch a pellet at 1800 or 2000 fps if small amounts of lubricating oil is on the side walls of the compression piston (spring or gas piston). This will make the gun very loud and the pellet may tumble.

[CrazyIvan]

I knew a guy who claimed he'd put a drop of ether starting fluid in the chamber. Never saw it done.