If you get a barrett, get the M99 single shot, it will shoot dramatically more accurately then any of the Barrett semi-autos.
I have shot several 50 cals ranging from the big McMillan BR rifles down to $1500 economy rifles. On the lower end of the price scale I would recommend the following. By lower end I mean under $4000. In no order here:
Armalite AR50
Barrett M99
LAR Grizzly
Serbu
All of these are very accurate rifles if fed quality loaded ammo taylored for each rifle. Of these, the easiest to shoot will be the Barrett 99 and Armalite AR50. The LAR is a great rifle and for a 36" barrel is amazingly compact, shorter then most conventional factory bolt rifles, but it is slow to use and kind of unstable for extreme range shooting. I have shot a 5.5" 3 shot group at 1000 yard with the one I used to own.
I now have an Armalite AR50 which will be rebarreled soon to my 510 Allen mag with a 34" Lilja SS barrel.
Of the factory single shot rifles I have shot, the Armalite is the most comfortable. Kind of ugly but hell, they all are to some degree.
The Serbu is a very light single shot, not terrible to shoot and generally pretty darn accurate and relatively low price, relatively!!!
It does have more recoil then all the others though.
If you want a 50 I would suggest you move on it soon. They WILL be outlawed someday and probably sooner then later. Also, if you get one, you will be very well served to load your own ammo. Your rifles performacne will be greatly improved!!
If you have any questions on the 50s drop me a line and I will give you all the information I have on them.
Good Shooting!!
Kirby Allen(50)