In the 1980s, the US Navy requested a new submachine gun to replace the MP5 then in use. In particular, the Navy wanted a gun that was optimized for use with a suppressor. H&K built two models of experimental guns in the 1980s to meet this request, creatively named the SMG and SMG II.
Both guns were hammer fired, closed bolt designs that used simple blowback actions. They had collapsing stocks similar to what would later be used on the MP7, and vertical front grips. The early pattern had a remarkably complex rear sight offering two different sets of range calibrations for subsonic and supersonic ammunition (out to 150m). The SMG II reverted to a more standard HK drum rear sight, but added a very interesting velocity-reduction system. A pressure vessel under the barrel was connected to the barrel via a set of holes just in front of the chamber, and when opened it would reduce chamber pressure and thus velocity. It could reportedly reduce muzzle velocity from 350 m/s (1150 fps) to 305 m/s (1000 fps), thus bringing it below the speed of sound.
The Navy tested both models, and decided that neither warranted replacement of the MP5. No other sales were made of the designs, but much of the developmental work would be put into the UMP program that came afterward.