A camshaft; the part of a typical engine that is timed with the crankshaft to actuate the valves.

"allow it" means to just leave something alone, or don't worry about it. People in West London say this quite often.

A piece of wood shaped like the stock of a rifle, with a horizontal bar projecting from one side. It magically turns semi-automatic weapons (those which fire one bullet per pull of the trigger) into bullet-hoses; a device which uses the recoil of a gun to fire the next round.
One of many ways to turn your shitty california-legal semi-automatic AR-15 into a machine-gun. Very popular in US states where semi-automatic rifles are legal; chiefly used by man-children and 13-year-old boys. On the down-side, it seriously impedes accuracy, and thus is useless for criminals.