In CQB, why use a handgun cartridge when you can use a real rifle cartridge?
Why use a rifle cartridge when you can use a shotgun shell?
If I could find one that was still working my first option would be one of these
High-Standard Model 10 semi-auto bullpup shotgun (12 ga)
Current battle or tactical rifle that I think is best suited for CQB missions is this one;
FN's F2000 5.56mm SMG