Like bang bang mentioned, there are alternatives if you want to get into .50 BMG shooting but you don't want to shell out full price for a Barrett. Bohica Arms and Spider arms both sell single-shot uppers that pin on to any AR-15 lower receiver. Armalite sells a single-shot precision in several calibers including .50 BMG, I think they start around $2500.
Remember a couple of things. THIS IS NOT CHEAP. If you have a rifle capable of 2000+ meter range, you must spend on real optics for it. NOT the bargain hunting scopes. You could EASILY spend another grand on the scope. You will spend a few hundred dollars on ammo just to get it zeroed and becoming familiar with it.
Also remember, this is not something that you can just pick up and learn instantly. There is a reason it's so ridiculous to claim that any terrorist can pick up one of these weapons and start doing serious damage. You learn this in stages. If you can't pick up a regular .22 with iron sights and make ragged 1" holes at 100 yards, you probably don't need to play with a precision .30 caliber rifle at 500 plus yards. If you haven't mastered the .30 caliber rifle, you aren't going to impress anyone with a .50 caliber rifle. Make sense?