From the Bushmaster web site,
Gov't. M16's can show throat erosion after as little as 2,500 rounds. However, that is with the lighter Gov't. profile barrel and always under full auto conditions. Under semi-auto conditions, those barrels will typically go 5 to 7 thousand rounds before breaking down badly enough to warrant replacement. Many times, a Gov't. barrel with 1 or 2 thousand rounds will be replaced as inaccurate, when what it really needs is to have the copper fouling removed from the bore. This is a tedious job, but the reward is a "like-new" barrel. This refers only to chrome lined barrels like ours or Colt's. Mil spec. SS109 ammo will not measurably increase barrel wear under semi-auto fire and our mil. spec. (chrome lined) barrel will outlast any sporting rifle barrel - period. More barrels are ruined from over cleaning - or careless cleaning - than are ever "shot out". Chrome lined barrels really only need to be detail cleaned when the groups start to suffer. Otherwise, a little powder solvent (or "Break Free" with CLP), and a few passes with a brush, clean the chamber well, dry everything off and apply a very light coat of "Break Free" or "Rem-Oil" and put it away. We have had barrels here go 20,000 rounds and still be within mil. spec. when treated this way.