Staff Writer (Updated: 8/20/2016):
Design work on the original MP5 product began in 1964 and spanned into 1966 while being officially attributed to the collective minds of Tilo Moller, Manfred Guhring, Georg Seidl and Helmut Baureter. The original working model was then known as the "HK54" to which this eventually became the "MP5A1" mark. Production commenced under the Heckler & Koch brand label in 1966 and, amazingly, continues even today (2013). Success of the series could be attributed to HK's use of its existing and proven automatic rifle components which became widely successful in previous company offerings at the military level. Basic construction of the MP5 involved use of steel and plastics for a lightweight end-product which was also compact - key qualities for any submachine gun. Basic models were available with simple single-shot and full-automatic fire modes while a three-round burst function was eventually introduced into the family line. Military variants then appeared with integrated suppressors fitted over the barren for reduced sound operation (to produce the "SD" - "Silenced" - line) and this version went on to be used with tremendous success, alone producing six major sub-variants under the SD designation. Various optics and accessories could be mounted along the receiver or forend of standard MP5s as needed, considerably broadening the tactical reach of the operator in-the-field. Basic iron sights were standard forward and aft.
While primarily chambered for the 9x19mm Parabellum pistol cartridge, HK eventually released MP5 models chambered for the 10mm Auto and .40 S&W. The standard 9mm form was fed from a 15-, 30- or 40-round detachable box magazine though other variants could make use of the 100-round Beta C-Mag ammunition drum. The action consisted of a roller-delayed blowback system utilizing a close bolt arrangement - the same as featured on several full-length HK automatic rifles - proving itself both accurate and reliable. Rate-of-fire, loaded/empty weights and effective ranges all varied depending on the chambering and production model in question.
The first definitive MP5 form became the MP5A2 with its fixed solid shoulder stock featuring single-shot and full-automatic fire modes through a clearly-marked selector switch along the side of the receiver. The MP5SFA2 was similar though with only a single-shot, semi-automatic firing capability. The MP5A3 brought about use of a collapsing shoulder stock with support for single-shot or full-automatic fire. The MP5SFA3 became a carbine version of the MP5A3 complete with collapsible stock and single-shot, semi-automatic fire. The MP5A4 utilized a fixed stock with a 3-round burst firing function while the MP5A5 was given a collapsing stock with burst fire mode. The MP5N was a specialized US Navy variant with Navy trigger group (showing fire selection through bullet symbols instead of the usual "SEF" text), collapsing stock and threaded sound suppressor support. The French Army received specialized MP5s as the "MP5F", featuring a collapsing stock and revised internals for their special ammunition requirements.