I’m not sure what the exact requirements are for a pistol brace, but they’ve all got a way to strap your forearm to it.
A pistol will have been manufactured as a pistol, with a short barrel and no stock. The flat surface area is irrelevant as far as I know. They’ve got rules as far as overall length, I think it’s got to be 26” total or less to still be a pistol.
And you’re allowed to shoulder a pistol brace now, so there’s not a lot of incentive to jump through the NFA hoops for a short barrel rifle.
As far as that AK, the recoil isn’t shit, but it’s pretty heavy so it’s hard to keep it on target without shouldering it.
Wifey shooting it, 7.62x39 out of a 7” barrel