I wonder if you could just bring a complete H-Bar upper to the store, slap it on to complete the rifle, and thus have it transferred as such? Not like you are missing out since it can't physically become a pistol anyway. But I am neither a lawyer nor an FFL.
Don’t know the law on that. You’d be doing the work and it isn’t like you can’t touch a gun until it is transferred to you. However I’d wonder if that would make the FFL a gunsmith in the eyes of the ATF. It would be changing the category the firearm is being transferred under from other to rifle at the very least (which is what you’d want). An interesting question though. Maybe one I’ll ask once they become available and I got to buy one.