As far as I know, the Eclipse barrel is the only one out there. I've heard of some .50 cal stuff from Bob Long way back, but I've never actually seen any of it. The GI Milsim stuff has its own barrel thread, so those won't work. I know a guy who made his own .50 cal barrel, but he has access to a lathe.
Another solution is having it bored for the .50 cal Freak inserts, but finding those are very difficult. I managed to get lucky and snag a set on eBay.