For the OP, a couple of random points:
At least with pointy bullets, in both the .300 AR's I have owned, they functioned reliably and shot better when loaded to mag length 2.24", rather than the AAC-recommended 2.08". Blunt tipped bullets, will likely require different OAL to feed well.
I was interested in pistol-caliber carbines for years, as a HD rifle, but that interest eventually morphed into a Daniel Defense V5 in .300 Blackout. It's a versatile rifle, and handier indoors, than a typical pump shotgun.
No, it's not a .30-30, and maybe not even a 7.62x39, but it does have some decent potential, with some good advantages.
Wilson Combat does sell AR uppers for a proprietary wildcat called the 7.62x40, which is basically a .300 Blackout stretched to mag length. They are usually designed purely for supersonic loads, and are very close to 7.62x39.
edit to add - Midway has 220gr Sierra Matchkings in stock, right now.
Last edited by tex_n_cal; 11/19/14. Reason: noted