I know nothing about the .300 blackout, but I'm quite familiar with the 7.62x39. I bought a couple of SKSs when they were real cheap, about 1990, along with a 550 round sardine tin of steel core ammo. Like most cheap ammo, it shot poorly as I recall.

I worked up many jacketed and cast bullet loads for the SKSs, and surprisingly, cast loads would often outshoot the jacketed ones. Finally lost interest in the incredibly crude but reasonably accurate SKSs.

I bought two of the limited edition Davidson Ruger MKII stainless bolt actions in 7.62x39 around '92 or '93. These guns shot just about everything accurately, including some cast bullets of over 200 grains, something I thought was not possible. When I finally concentrated solely on the cartridge, I tried to figure out what it was really good for. Despite what some say, it's ballistically not the equal of the .30-30.

For deer hunting, there are far better cartridges. Maybe it's niche is in the dreams of the tactically-oriented metropolitan mercenaries. I never really came to a conclusion as to usefulness. But, for the handloader that likes to experiment with different cartridges for the sake of experimentation, the 7.62x39 is as good a candidate as any other.