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How a HK UMP works
As originally designed, the UMP is chambered for larger cartridges (.45 ACP and .40 S&W) than other submachine guns like the MP5, to provide more stopping power against unarmored targets (with slightly lower effectiveness at longer range) than the MP5 (largely offered in 9×19mm, albeit with short-lived production of 10mm Auto and .40 S&W variants). A larger cartridge produces more recoil, and makes it more difficult to control in fully automatic firing. To mitigate this, the cyclic rate of fire was reduced to around 600–745 rounds per minute (RPM) for the UMP40 and 600 rounds per minute for the UMP45, which makes it one of the slower firing submachine guns on the market.
The UMP9 (the 9×19mm version of the UMP) is almost 0.2 kilograms (0.44 lb) lighter than its MP5 counterpart. Its predominantly polymer construction reduces both its weight and the number of parts susceptible to corrosion. It has a cyclic rate of fire of around 600–650 rounds per minute.
The UMP is available in four trigger group configurations, featuring different combinations of semi-automatic, 2-round burst, fully automatic, and safe settings. It features a side-folding buttstock to reduce its length during transport. When the last round of the UMP is fired, the bolt locks open, and can be released via a catch on the left side. The standard viewing sights are composed of an aperture rear sight and a front ring with a vertical post. It can mount four Picatinny rails (one on top of the receiver, and one on the right, left, and the bottom of the handguard) for the attachment of accessories such as optical sights, flashlights, or laser sights. Vertical fore-grips can be attached to the bottom rail for increased control during burst and automatic fire.