As I have asked before, why do you want a carbine based on an underpowered pistol cartridge, and a 9mm is pushig it for handgun SD? It makes good sense in an SMG. It makes little sense in a carbine unless you want accuracy without penetration. Otherwise you get all the concealability of a rifle and all the power of a pistol. Rant over.
Serious advice. I am no great fan of kel-tec, but they do have a pistol carbine that makes sense. Their SU2000 (I think). The idea here is to build a two gun combo with interchangeable mags. You choose your caliber, 9mm, .40 etc, then choose your brand, Glock, Sig etc. The rifle will accept the pistol mags. This strikes me as a useful and innovative idea. I wish someone other than Kel-tec had devised it but lots of folks love them. Its like the old .45 Colt rifle/pistol combos of the west. Not ideal, but a practical SHTF option.
FQ13