re: Official AR-15 thread

Carburized carriers didn't have wear problems in the bolt chamber that through-hardened carriers did. That was before Colt hard-chrome plated that part of the carrier though. Wear resistance and durability were cited as reasons for the switch to Parco-lubrite from electrolized plating. Part of that durability comes from the required shot-peening of the parts prior to coating. I haven't found much on the decision to use C158 for the bolt vs other materials though there was a request to drop the RC hardness of the bolt from 60-61 down to 57-58 which resulted in a 30% increase in the average number of rounds before failure.