I don't know how public this info is, but I was talking with someone at Barrett this past week and he informed me that they have contracted IMI to make .50BMG ammo for them. Comes with 'Barrett' stamped on the head and everything.
Price quoted was approximately $240 per 80 rounds boxed, but verify that for yourself. I was told that the accuracy using them was better than anything else they have tried in their rifles.
I didn't see it listed on their website, but I may have just overlooked it.