please refer to ‘gearbox'

The central mechanism of an Airsoft AEG Every AEG has some form of Mechbox inside.  It translates the rotational movement of an electrical motor in to a reciprocating motion for the piston, using a set of gears.  The mechbox also loads BBs in to the breech, powers each shot and provides the Safe, Semi and Full automatic functionality. The mechbox is also known as a Gearbox by some users, although as the gears make up so little of this device, Mechbox is the more commonly used alternative.