Consider the AR-15 in AAC 300 Blackout. Just a barrel change away from 5.56 or two pins away if you have a complete upper. I put a complete upper together over Black Friday for $600. Ammo from Washougal River at $175 for 250 rounds. Even the mags are the same. The AAC 300 Blackout has been a SAMMI standard for some time, not a wildcat.
Roy