I run a Nightforce 5.5-22x56mm on my AR-50, if you can wait and save up just a little more you could pick up a NF. The scope has a 30mm tube with 100 MOA built into it. This will get you out past 1000 yards w/o any problems, if you use a mount with MOA built into it you'll be able to get out even further.
If you don't have any place to shoot beyond 1000 yards I would just mount it on top of the rifle if it has a flat rail. If you do plan on going out past 1K then look into getting either a 20 or 30 MOA rail. Either one will take you out well past 1K yards.
If you don't want to spend that much, look into the higher end Leupolds as they have great customer service, lifetime warranty... Just be sure that you also get a scope with enough eye relief, you'll want a minimum of 3". Any shorter and you run the risk of getting scope eye and that ain't fun when you're talking .50's.
Go over to
www.longrangesupply.com and check his prices, he also posts on Biggerhammer.net as Don@long range supply. He posts discounts there for the board members as that's where he got his start years ago. Haven't heard a bad word about him.