I had an ATN on my 50 and it was heavy. I didn't particularly care for it but sold it with the rifle...
I like Vortex on my std rifles but can't say if their rated to hold up on the big gun... they have a good warranty so i'd give one a try.
I second the PST recommendation.
Terrifically expensive. Please tell me how much you spent on the Barret? A good rule of thumb is, the price of your scope is going to be half if not as much as the rifle itself. You want good. You pay for good.