I don't want to throw a monky into the works but a quick question...
didn't the .50 calibers on aircraft and the .50 calibers on Allied equipment have different types of barrels.  
i think the aircraft ones had the perferated air cooled barrels and the ground vehicles ones had 2 or 3 types with the changing handle on them near the base of the barrel so they could be changed when they were hot and a couple of types without the changing handle.. 
though I'm sure there were many aircraft types that found their way into ground equipment...  
