My understanding regarding the .50 BMG is that there isn't much good ammo out there. IIRC, a lot of military .50 ammo tends to be around 6 MOA. Rifles like the M82A1 are really more for disabling equipment and vehicles than for shooting at man-sized targets. (Their weight makes them somewhat subpar as a standard sniper rifle.)
I'm not sure there's any match-grade ammo, though I could be wrong. Of course, handloads might do great, but I'd be really sure I knew what I was doing when handloading for that mother.
If you want good groups with a lot of power, you might also consider the .338 Lapua. I'm thinking of a .300 Win Mag or 7mm Rem Mag myself for a rifle with 1000-yard range, then putting in a lot of practice.
Oh yeah, then there's the .50's recoil. I've fired bolt-action and single-shot .50s. My size is 6' 2" and ~210 lbs, and I'm not very recoil-sensitive, but let me tell you: those things pack a wallop! For some perspective, the free recoil energy of a 30-pound .50 cal rifle is around 100 ft-lbs. That's about the energy you'd soak up by being shot with a small handgun at close range. Obviously it won't do the same damage, but it will give you a good jolt, and that will take some getting used to.
One big point in favor of getting the .50 is simply this: Since most gun owners don't seem to want to (physically) resist the theft of their rights, .50s WILL eventually be banned or severely regulated. If you really want one, then now's a good time.
I've not fired a Ferret, but I have fired a Serbu. It seemed to be of very good quality. I'd probably take one of those over the Barrett and then spend the money saved on other rifles, ammo, optics, etc.
Good luck