I guess I have to plead ignorance in not keeping up with current regulations. In my understanding when the AR pistol came out, it was a felony to shoulder it because it was a SBR needing a class III ticket.
I still think it's a work around, but if it's legal, go for it.
That's gone back and forth and is not currently cause to consider a braced pistol an SBR.
Regardless, I don't shoulder my AR pistol. I brace it against my upper chest, too close to the centerline of the body to be considered anywhere near the shoulder pocket.