It is a large air cooled machine gun used by German forces. Nicknamed 'Hitlers zippers' because its high rate of fire caused it to sound like a zipper or cloth being ripped. It could literally shred a man.
Movies alway show it with a slower rate of fire typical of allied machine guns.
An almost identical gun is still in use by Bundeswehr forces today. The US has always had lousy machine guns. The current model is an abomination.