Staff Writer (Updated: 8/29/2016):
The MP5K design was decidedly Heckler & Koch with strong clean lines and the requisite black finish. The weapon featured a deep receiver containing the required internal components and a shortened forend housing the barrel assembly. The pistol grip and trigger unit were integrated as a single unit with the easy-to-read fire selector set over the thumb area. There were three modes of fire - Safe, Semi-Automatic and Full-Automatic, these marked as "S", "E" and "F" respectively and actuated with a simple flick of the selector lever. The MP5K featured an optional collapsing shoulder stock to retain its compactness yet offer up a conventional "three-point" support as needed. Additionally, a forward vertical rubber grip was set ahead of the magazine well as there was a no handguard present. The magazine well accepted the typical curved HK magazines containing 15 x 9x19mm Parabellum cartridges (this could be substituted with the more traditional 30-round magazine as needed). A drum-type iron sight was fitted to the rear of the receiver with a hooded front post above the muzzle. Internally, the HK MP5K was largely a copy of the full-size MP5 submachine gun before it. In an effort to keep the design compact, the barrel, cocking handle, bolt, receiver and primary (15-round) magazine were shortened though, despite its length, the receiver of the HK MP5 could be adapted to accept various optical mounts. However, its lack of a protruding muzzle meant that no muzzle attachments could be fitted.
The MP5K line was expanded to include four distinct models beginning with the base MP5K itself. The MP5KA1was completed with a smooth upper surface and smaller iron sights to prevent snagging when drawing the weapon from a holster (in the machine pistol role for example). The MP5KA4 was given a three-round burst capability while the MP5KA5 was similar to the MP5KA1 though with the same three-round burst capability of the MP5KA4. The MP5KA5 also lacked the shoulder stock of prior marks. The MP5K and MP5KA4 were also given open adjustable sights while the MP5KA1 and MP5KA5 were finished with fixed open sights.
The MP5K line was broadened in 1991 with the arrival of the MP5K-PDW - "Personal Defense Weapon". This mark brought about more commonality with the full-sized HK MP5A2 submachine gun. It featured an optional folding stock constructed of synthetic materials for a lightweight - yet durable - finish. The Personal Defense Weapon designation afforded it the qualities of a full-automatic firearm with the compactness of a machine pistol and, therefore, no drop in the level of firepower being brought to bear.
The SP89 was a civilian market variant of the HK MP5K though limited to semi-automatic fire. This version also lacked the identifiable vertical foregrip, this now being replaced by a handguard.
The HK MP5 was respectfully known as the "Room Broom" for its high volume fire and compact size.