You have all kids of options in the AR15 and AR10. The AR10 can do any of the 308 family cases (243, 260, 7mm-08) along with the 6.5 Creedmore, 6XC, 6mm-250, etc. Just need to get a barrel chambered for what you want and you're off and running. AR-15s same way. The 6mm variants are better...