Grease guns are used to apply viscous lubricant onto moving mechanical parts, used commonly in machine shops and garages. Properly lubricating moving metal parts helps to increase the service life of machines and avoid abrasions. Grease lubricant is applied with a grease gun, available at most hardware and auto parts stores. Loading one can be a bit messy, but uncomplicated, whether you've got a cartridge loader or a reservoir-style grease gun.