Looks very short for a flak gun casing. has it been cut down?
So the concept of the American Mk19 is a remake of the WWI German m3.7 Flak Maxim... What was old is new again.
They have one of these guns on the lower floor of the Canadian War Museum. It looks very impressive, but from what I've read, the actual performance was not particularly well suited for anything. Low velocity, slow rate of fire, and not much explosive power in the shell.