Staff Writer (Updated: 7/25/2016):
The principle feature of the MPSD series is its large aluminum suppressor assembly added to the front of the weapon which is coupled to base supersonic 9mm cartridges. Within this assembly is a shortened barrel that has been perforated with some 30 openings to allow for the controlled escaping of gasses as the weapon cycles through its action. The suppressor therefore shrouds the perforated barrel assembly and is designed with a two-stage, two-chamber process. The initial chamber (the one closest to the receiver) surrounds the barrel in an expected fashion and it is this chamber that initially collects the escaping propellant gasses, controlling the effects of the escaping bullet by reducing its pressure and retarding its acceleration. The second chamber (ahead of the muzzle) then takes these gasses and nullifies their effects even further by increasing gas volume and reducing its temperature while allowing a slow escape. The end result is that the subsonic bullet exits the muzzle at a lowered, now-subsonic, velocity and thusly its audible signature is reduced. Due to the fact that the MP5SD does not make use of subsonic ammunition coupled with its suppressor, it is not a truly "silent" weapon in the accepted sense. A lightweight bolt assembly does figure into the device, however, and helps to lower the audible signature of the internal action.
As with other variants in the MP5 production line, the MP5SD series begat a small host of sub-variants based on the initial design. The MP5SD1 was the initial silenced offering and lacked a stock (an endplate fitted in its place) while utilizing the standard semi-/full-automatic fire selector. The MP5SD2 was fielded with a solid fixed stock and same trigger group as the MP5SD1. The MP5SD3 sported a collapsing stock but was otherwise the same as the SD1 and SD2 before it. The MP5SD4 lacked a stock but instituted a burst-fire function. The MP5SD5 was similar only given a full solid stock. The final SD model in the series became the MP5SD6, complete with a collapsing stock and three-round burst facility.
A specialized version of the MPSD3 was ordered by the United States Navy as the "MP5SD-N" and given a collapsing stock, "Navy trigger group" (visual bullet labels as opposed to the original "SEF" letter labeling), larger suppressor of stainless steel construction (by Knight's Armament Company) and a tritium-illuminated front post sight. This version (available with collapsing stock - MP5SD-N1 - and fixed solid stock - MP5SD-N2) has been used by Navy SEAL and SAS operatives.