I have not thoroughly researched this...this is just off the top of my head...so somebody please correct me if I am wrong...The K-40 is a full length slide and full length frame...The MK-40 is a compact slide and compact frame...The K-40 covert is a combination of both...it is a K slide with a MK frame. The benefit of the covert over the MK is longer sight radius, slightly higher bullet velocities due to longer barrel, and possible greater reliability due to longer slide cycle length/time. The benefit of the MK over the Covert is greater concealability. The capacity of both is the same. The MK would be slightly lighter than the Covert. I have a K-9 Elite 98 and love it. I have been most impressed by every Kahr product I have shot or handled. I don't think you will go wrong with either.