H&K introduced the UMP (universal machine pistol) in an effort to attract sales from American law enforcement by offering a modern submachine gun in .45 ACP and .40 S&W calibers. The MP5 was and remains an extremely popular SMG, but it has never been made in .45 caliber, and that was seen as a popular option in the United States. The UMP was designed as a thoroughly modern gun with polymer upper and lower assemblies and a blocky, easy-to-machine bolt. It does retain the closed bolt operation of the MP5, providing much improved single-shot and first-shot accuracy compared to most SMGs. It was designed initially for .45 caliber, and then barrels, bolts, and magazines followed for .40 S&W (which was popular at the time) and 9mm Parabellum. The rate of fire in .45 is relatively slow (about 500 rpm), although it is faster in 9mm (more like 600 or 650 rpm). Sales have been primarily in the larger calibers in the US, but a number of security agencies in Europe have purchased UMPs in 9mm.