The UCP operates by roller-delayed blowback. The external design of the weapon appears to have been borrowed from the P2000, including ambidextrous controls, interchangeable backstraps, and a MIL-STD-1913 Picatinny rail for accessories. Similarly to the P2000 and the USP pistols, the UCP's trigger mechanism was modular, making it capable of different configurations. The weapon could accept an extended, threaded barrel capable of equipping a silencer made by Swiss firearms manufacturer Brügger & Thomet.
It was designed to be the companion sidearm to the MP7 PDW, using the same 4.6×30mm bottlenecked cartridge. In this sense, the UCP was a competitor to FN Herstal's Five-seveN in the same sense that the MP7 rivals the P90.
The design remained in prototype stage as of 2006, and was reported as entering limited trials with the Bundeswehr.
H&K USA president Wayne Weber announced in July 2009 that the UCP was canceled, with the reason being that "HK felt it did not provide adequate ballistics in handgun form."
- modernfirearms.net article on the UCP