Wow! Excellent info! I've owned and shot a previous (RRA) AR-15 in .223, and I've owned and shot both SKS and Sar-1 AK-47 in 7.62x39. I've just never had an AR in 7.62x39.
The question stems from a long-time interest in having a harder-hitting round than .223/.556, that is still quite affordable to shoot and that works well in the AR platform. I know the answer is to hand-load, but it would sure be cool if there was a factory round in the 6.5 to .300 range that was inexpensive, plentiful, AND worked well in a standard AR action. The 6.8 SPC II was my favorite until I read that you really need a dedicated upper and magazines, which is also true of the 6.5 Grendel I hear. And neither of those are offered in a particularly cheap factory ammo. I learned that 6.5 Creedmoor is a much more ballistically robust offering, but that it is a long-action round requiring an AR-10 type platform. And again, not especially cheap to shoot. Other than the big bores (which are not cheap at all), that leaves only the 7.62x39. To be really affordable though, you'd probably have to shoot steel-cased ammo. Hence the original question. For now, I guess the .300 Blackout that I opted for is a good compromise, and I plan to build a .223 upper that I can swap out for even more inexpensive shooting :-)