Anyone rebarrel an AR50 to 375x50bmg?

You'd be better off shortening the case to something smaller like the 416 uses.

I haven't, however I was involved with the 375 Warner development about 10 years ago and we ended up cringing at each shot due to the incredibly short barrel life. It wasn't a 375-50 but more like a modified 375-415 Barrett with a different shoulder. Powder availability is the problem we ran into and I'm not aware of anything new in that realm which would be appropriate for such a case.

The 416 Barrett and 416 Warner are both similar needs for powders. The shortcoming of all of these things is still the brass for all of these various 50 and 50 based wildcats. It's extremely expensive brass and it was never designed to operate at the 60+ ksi pressures that we try to shoot them at.