The M249 Squad Automatic Weapon (SAW) is a light machine gun used by the US Armed Forces and probably the most complicated gun MOC I've ever constructed. The life-size (1:1), purist replica features a lifting cover, pull-back bolt handle, ambidextrous safety switch through the grip, a working bipod, flip-up wire shoulder support, and accepts either an ammo-belt box or M16-style magazine.
Last, but not least, the whole thing is motorized! Pulling the trigger actually turns on an internal motor and vibrates the ammo belt to produce a realistic feeding motion/sound. See an old video of it here: https://youtu.be/uaI60CooVoE