The MP5 is arguably, next to the AK-47, the most popular firearm in the world. Because it is so common in close quarters combat and police situations, it has found its way into many games that feature such.
It is a submachine gun designed by the German firearms manufacturer Heckler & Koch after the success of the then-new G3 assault rifle. Derived from the G3, the MP5 entered production in 1966 and is still being produced today, with no future plans to end production. The most common variants fire standard 9x19mm Parabellum ammunition at roughly 800 rounds per minute. Other variants fire 10mm Auto and .40 Smith & Wesson. While originally using straight double-stacked magazines, all models now use curved 15 or 30 round magazines.
Due to its age and popularity, a plethora of variants have been produced for different purposes. Most differ mainly in trigger groups, the most common and generally seen in games being the Sicher (Safe)/Einzelfeuer (Single fire)/Feuerstoß (Continuous) (SEF) or Navy group. Only two, the MP5/10 and MP5/40, differ in ammunition, both of which were discontinued in 2000. The only variant which differs noticeably from the standard is the MP5K (Kurz, "short" in German), which is meant as a personal defense or special services (e.g. VIP protection) weapon.
The MP5 is used by the U.S. Navy SEALS, British SAS, German GSG9, and dozens of other military forces around the globe. It is even utilized by many law enforcement agencies, especially in the U.S., Germany, and Turkey.
However, recently with the advent of carbines and shorter barreled subcarbines that fire more effective rounds, the MP5 is being replaced with carbines.