Shove . . .
Get a good one, meaning, "accurate", they really only shine at very long range, there are many more smaller caliber rifles with excellent accuracy that do a fine job inside 1,000 yards or so. The great BC of the .50 bmg slugs (A-max is 1.050) lets the really get out there with minimum wind and reasonable drop, but that's all wasted if you're shooting a 2 minute rifle and can't keep them on target.
Most of the more 'fun' ammo isn't very accurate. I've had best results with straight "AP's" in milsurp and with Hornady A-max's for match ammo. Barnes makes a solid bronxe "bore rider" that is supposed to be quite accurate too. You really aren't getting the most out of your rifle unless you're handloading - everything is different, even if you already handload, it's cost you big to get started up with the .50 - maybe $500 . . . .
Barretts are nice rifles, but accuracy can be lacking, especially the M82 series. "ALS" makes a bolt-action upper for an AR15 that comes in a variety of configurations. "State arms" makes an 'ultralight' rifle that gets good reviews too. I've shot an older "Watson's Weapons" upper that shoots like crap and is aggravating to operate, they say the newer ones are better, a little.
The FCSA holds fifty caliber matches at Quantico a few times a year. It's between 1-1/2 and 2 hours from BWI with average traffic. I'd suggest going down there and watching them shoot and checking out the gear.
Some .50 links for your amusement:
General info:
http://forum.fcsa.org/
http://www.biggerhammer.net/
http://www.50bmgstore.com/
Custom dies:
http://www.ch4d.com/
http://www.corbins.com/
Good luck!