biggest issue with ARs in 7.62x39 are magazine feeding issues, and bolt failures. You can fix the magazine issue with the right lower, but you can't fix the bolt issue with a standard sized AR15 sized rifle. This is why the CMMG mutant came along. Takes AK mags, and uses the AR10 sized receiver so you can upsize the bolt and eliminate the problems there.
AKs in 5.56 do work, and countries out there currently use them.
oh, and don't leave out my favorite, AK74 ie 5.45x39
AKs in 5.56 do work, and countries out there currently use them.
oh, and don't leave out my favorite, AK74 ie 5.45x39