Hi, Frogman,
I haven't seen that one. I guess one could make a .50 BMG upper for an AR-15, but I wonder what the recoil would do to the alloy lower. (Not to mention what it would do to the shooter!) It would have to be single shot unless it has its own mag someplace.
Let us know if you find one and how it works.
Jim
P.S. I wonder if the .50 spotter round for the 106 Recoilless would work through the magazine?
JEK