I dont know if it staged or not , I had to listen to several times but it sounds like as he is standing there holding the gun in one hand he says "no I ain't gonna do that" , he then goes to the 2 hand hold and the guy behind the camera says "You big sissy"
the he fires and you here say " holy F%$#."
I dont know anything more than you do about the caliber and I have no idea of the origin of the video but I found it entertaining