Well, kinda yea:
7.62x39 is too tapered to feed well in AR magazines, notice that an AK-47 magazine is much more "curved" than any M-16 magazine. Stuffing the one in the other would lead to problems.
The other factor is quality of bullets: most 7/62x39 is of low quality projectiles and components. Yes, you can get better. But even quality projectiles that have modern expansion like round nose, ballistic tip, hollow points, are almost unavailable to that cartridge of .311 diameter. BUT... the .300blackout is a standard .30 caliber projectile, for which there are hundreds of different quality projectiles of all shapes and sizes.
So really it comes down to feeding in large cap AR mags, and wide selection of projectiles.