The M99 is a very accurate rifle on average, in fact Skip Talbot used this basic platform to set a world record here a couple years back in the light class at 1000 yards.
The biggest problem I know of with the M99, at least the eariler ones was the triggers as they were pretty heavy and creepy. THis was an issue Barrett was reported to be working on so this may not be a problem any more.
TO be honest, for the rifle you have, the 750 gr bullets will be very hard to beat. It is accurate in most rifles, very good B.C. values and is legal to use hunting in states where the 50 BMG is legal for use because it is an expanding bullets. There are several other bullets on the market but for a good combo of accuracy and ballistic performance the A-Max is pretty hard to beat. Its also not to overly priced like some of the 50 cal bullets out there.
One bit of advise, if you are not set up to load for the 50, get that way. YOu will be able to shoot your rifle dramatically more if you roll your own and your groups will improve dramatically. You will pay around $5 per round for quality match ammo preloaded. If you roll your own it will run you less then half that once you pay for your brass to load true match grade ammo.
Practice/plinker loads can be loaded for just over a buck a round once you have the brass.
For practice rounds, I would recommend getting some of the pulled Mil Surplus projectiles, I purchased 500 700 gr AP bullets s few years ago basically because they only cost me $130 for the 500 of them. These are not know for real accuracy, in fact they are designed to have a certain dispursal factor built in for use in machine guns.
I found if you sorted them out, especially sorting out the ones with loose cores(mild steel cores) they shoot suprisingly well out to the 1/2 mile mark for casual rock busting. A 700 gr Steel cored armor piercing round really makes a pop when it hits solid rock out to any range, very fun gravel maker and good practice as well.
DO NOT USE THE TRACER BULLETS. YEs these are fun but they are not recommended for match grade barrels and pose a fire hazard as well so they are not worth playing with in your fine barrel.
You can go to www.wideners.com
and get a pretty good selection of 50 BMG bullets, brass and other things for the 50 BMG.
Their last add in Very High Power listed the 700 gr AP surplus bullets for $126 for 500 ct with prepaid shipping.
They also list the 750 gr A-Max at $23.00 per 20 which is about as low as I have seen them for quite some time.
As far as brass goes, by new hulls, not surplus stuff. These cases are cheap as hell but require alot of prep time to load for. I highly recommend either PMC or IMI brass. this brass is brand new and will run around $1.00 per case, not to shabby compared to the likes of Wby, Lazzeroni, Norma or Lapua brass.
Powder wise, H-50BMG is very hard to beat for match grade ammo but there are several powders on the market that will wrok well at lower prices, AA8700 will also work well and is much cheaper then the Hodgdon powder. IT is a dirtier burning though in the 50 BMG. Also there are surplus powders out there like WC872 that are great powders and very cost effective, around $40 per 8 lb keg.
You will go through alot of powder so it is better to buy it in bulk.