Bolt
The handle that cocks / charges / reloads a gun
The bolt is present on most rifles and SMGs, both real and airsoft. When manipulated, it allows the user to pull back the slide hidden inside a gun and load a fresh bullet or BB. A bolt is unnecessary on guns where the slide is readily accessible (such as semi-automatic pistols).
NOTE: While AEGs have a bolt, the mechanism of the AEG at this time means that the bolt has no function. It is entirely cosmetic in all but a rare few AEGs.