not a .50 BMG. It looks like the Mark 19 grande lancher.
Actually it is neither. They tested with both but it is a different machine gun all together used solely for tail gunning on 53's I guess. It was just getting incorporated when I got out. The gunner is my buddy he gave me this pic shortly after I got out, and then I found it again on Google.
Since you have a friend than plays with it then I will take your word for it than. LOL Semper Fi
Slightly smaller with a much smaller round. LOL