Depends on game modes. If it's HCTDM, I'll go with B23R. If not, I'll go with the KAP-40, especially dual wield. It's just that the B23R is EXTREMELY accurate, and out-shoots even assault rifles.
Honestly, it depends on what you play. If your a hardcore player like myself then the b23r is a better choice in my opinion because since its burst you don't take the risk of over firing when the persons already dead like you could with an automatic. And since its burst you have a much better shot at range but that could easily be argued if you tap-fire.