The only comment I can make on that since we don't know the origin of the comment is that I have only been involved in one instance where helicopters had MG's fitted into the cabin. The mount and guard rails (I think the RAF fitters called them 'taboo tracks' for some reason) have to be absolutely foolproof and therefore somewhat complicated. There must be absolutely NO chance whatsoever of the bullets hitting the rotors, even in the most extreme circumstances, for obvious reasons. And this requirement must extend a bit further than telling the door gunner '.........don't shoot above the door line' because when you're shooting and the thing is banking, you don't know where you are!