Colt made some ARs in 7.63X39, but discontinued them due to mag feeding problems. I'm building an AR in .300 Blackout next week. It should perform nearly the same as the 7.62X39, but uses the 5.56 case as a parent. Same mags, bolt, everything as the 5.56 AR. Simple barrel change is all that's needed. I'll be running an Osprey GP conversion on mine to negate the need for an adjustable gas block, as I will be shooting both sub sonic and super sonic ammo.