Mattel's M-16 Marauder was a full sized replica of the real thing. When you cocked the rifle and pulled the trigger the Marauder let loose the sound of machine gun fire. The more you cocked the gun, the longer the burst.
Trivia: In John Wayne's pro Vietnam film "The Green Berets", there's a scene where Wayne smashes his M-16 against a tree. It's said he used Mattel's Marauder instead of the real thing.(source)