I don't know about the Hex Mags, but read complaints about Pmags and some about Lancers (love 'em for 5.56). THE D&H teflon USGI variation is designed specifically for the round. They are stamped 300BLK. I've only gone though a few. No probs yet.

http://shopwilsoncombat.com/AR-Style...HMAG30-300BLK/