Depends on what you plan on using them for....

You should be able to load up to and including 69 gr Sierra Match Kings or 68 Gr Hornady Match bullets. Some 1:9 barrels will also handle 75gr Hornady AMAXs as well. I've used the 69 and 68 gr bullets in my 1:9 Bushmaster barrel- it preferred the Sierras, but never loaded anything heavier in it, the barrel also shot 52gr SMKs very well also. I just rebarreled to a 1:6.5 Pac-Nor so I can shoot 80+gr bullets for x-course competition.


Edit: One more thing, if you haven't reloaded for an AR before, watch your cartridge headspace. Buy a RCBS or Stoney Point (I forget who they are owned by now-maybe Hornady?) cartridge headspace guage ( I use the stoney point) to measure case stretching and set up your sizing die. You absolutely have to FL resize your cases if you want the cartridges to chamber every time and more importantly to avoid a slamfire, but you only want to set the shoulder back by 0.002" toavoid casehead seperations and extend brass life. Factory ammo tends to be on the very short side and some dies are even shorter. Out of the bushmaster barrel, I was typically getting 0.004" of difference between fired and unfired brass, and an extra 0.003" if I 'cammed' the die over per manufacturers instructions if I didn't set the cartridge headspace the right way.