I think what he meant by black market UMP is the MP5 RAS. This gun is considered a better version of MP% because it has less recoil and more damage. It is just like the M4, almost no reocil at all. And what he meant by hitbox problem maybe he shot someone from the back (they haven't fixed the back hitbox yet) and he didn't hurt him at all. With much more recoil gun, it will actuallly spray and may hit the hitbox.