Spend a little time studying on the bolts and bolt carrier groups, don't buy the lowest priced bolt or bolt carrier group. Bolts for AR's are built of Carpenter no. 158 and 9310 steels; the 9310 bolts are hardened completely through and the carpenter no 158 bolts are case hardened. the carpenter no 158 bolts are less likely to break for this reason, they are more costly to be manufactured, Colt and a few others manufacture them. Pay attention to the pressures closely in your AR's, they won't handle the higher pressures of some of the top end loads that reloaders load. There is an interesting article by Gun Builders Depot the full guide to the AR-15 bolts carrier group, although it says the complete guide it isn't, but it does have very good information in it. When you get into building a more accurate AR there are things done that the average person really doesn't need to be concerned with that those builders do just like reloading for competitive shooters do to their brass bullets and powder measuring. It can be a very complex world to get into or just a matter of looking into it far enough to build a safe and fun AR.
