300 blackout may use a 7.62mm diameter bullet, but a 7.62x39 case looks nothing like a 300 blackout case ... the dimensions are totally different and they are not interchangeable. The chamber on the x39 is ~0.065" larger in diameter than the 300 blackout case at the base ... it would more than likely just blow out.
I don't have a drawing of the blackout case in front of me, so I'm not sure if it would chamber ... it might, but even if the round chambered and fired, it would be very dangerous ...
Unfortunately, the .30 of the .300 Blackout isn't the 7.62 of the X39, so even the barrel is problematic.
The number games of cartridge makers starts the problem, and we live with it. Kinda like shooting a .38 in a .357. Looks like a numerical disaster, but you can.
Gun stuff isn't clear and simple as it looks. Get into cars, knives, watches, and flashlights, you bump up against stuff like that all the time.
