The German company Heckler&Koch created the USP, also known as the "universal self-loading pistol," to replace the P7 line of pistols. The USP is a short recoil-operated semi-automatic handgun with a mechanically closed breech. A large rectangular lug above the chamber of the barrel serves as this lock-up mechanism's key, riding and locking into the slide's ejection port cut-out. Most of the design specifications for the USP came from features that U.S. law enforcement and military operators respect. The controls are utterly American, thanks to influences from well-known designs like the Government Model 1911 handgun. All USPs employ a fiber-reinforced polymer frame strengthened in stress and friction-prone locations by stainless steel inserts. With the creation of the VP70Z and P9S pistols more than 30 years ago, HK was the first to utilize polymer materials in the construction of handguns. The mechanical recoil reduction mechanism is one of what makes the USP unique. It includes a short extra spring that is tucked into the primary recoil spring at the recoil spring assembly's breech end. The mechanism minimizes the maximum recoil pressures experienced by the shooter and was created mainly to increase the durability of the pistol's components. This recoil reduction mechanism has been tested and proven in the HK Mark 23 pistol designed for the U.S. Special Operations Command. The USP is offered in three calibers (.40 S&W, .45 ACP, and 9mm; additionally .357 SIG only for a compact version), DA/SA or DAO modes, and nine trigger/firing mode combinations.