The mag well on repeating 50bmg uppers is at 9 o'clock rather then 6.It'd be single shot only since there is no way in hell you'll feed BMG through the AR15 or even AR10 mag well... Unless you're talking Beowolf then that's a different story.
There is a single shot 50BMG that uses the same ergonomics (same grip and safety is in the same place) which I think is an Armalite product if I'm not mistaken but it's all proprietary.