300 ACC Blackout is a 7.62 x 39 for AR's. Super SHTF round, but not popular enough yet. I might get an upper for fun.
Actually the 300 ACC is based off the 300 Wisper and was designed to run subsonic with a supresser and heavy bullets. It does fairly well supersonic with lighter bullets as well but is in no way made to compete with the 7.62x39. The 6.8 SPC is more comparable to the 7.62 but doesn't have the feeding issues in an AR platform associated with the heavily tapered 7.62 cartridge.
You're both right for the most part; the .300 AAC Blackout is designed to run .30 caliber rounds in a 5.56 platform with all parts being interchangeable except for the barrel. You can either run it with subsonic ammo and a suppressor, or use supersonic loads to mimic the ballistics of the 7.62x39mm. The supersonic .300 Blackout and the 7.62x39mm are very similar ballistically, and both are different from the 6.8 SPC.