I shouldn't comment here...but that's never stopped me before, so...
I do not understand the desire for a pistol-caliber carbine. I believe such things to be purely military weapons, and then only in a full-auto configuration.
The pistol-caliber carbine is a back-up weapon when your normal gun is, maybe, a sniper rifle, a 75mm cannon, or a .50 M2 machinegun. It is meant to be used by an assistant gunner for repulsing a flanking attack by infantry, when the gunner is too busy or involved to swivel toward the new threat.
If you're going to the trouble of carrying a somewhat-long gun (that is, a carbine), why not carry one in an effective caliber like .223 or 7.62x39mm? Even .30 US Carbine is a better cartridge than .40, in a carbine.