If I am going to shoot 7.62x39, I am going to pull out the AK-47, which is by far the best weapon system for this cartridge.

If I am going to shoot a subsonic AR-15 SBR, I am going to use 300 Blackout or possibly 9mm.

I don't see a lot of usefulness with an AR-15 chambered in 7.62x39; personally, I would rather cut down the 5.56 brass and use a non-tapered cartridge in this weapon system with the correct dimensions on the bolt carrier. I think it also comes down to the availability of re-loadable brass. In the last five years, I picked up over 4 five gallon buckets of free 5.56 and 223 brass. In that same time, I picked up one 30 cal ammo can of re-loadable 7.62x39 brass.