There are a fairly wide variety of silenced Sten guns that were made during World War Two, because many were needed for small Special Operations Executive missions. However, the British Army did also formally develop and adopt such a weapon. It was initially requested in 1942, with the first trials in November of that year. After two years of tinkering and deliberating, a pattern was finally put into production in February 1944. This was a Sten MkII with an integrally suppressed barrel. The barrel was just 3.75 inches long, with six vent holes drilled just in front of the chamber to reduce muzzle velocity below the speed of sound. The silencer itself was about 12 inches long, with an initial expansion chamber and 18 baffles.
Since the vented barrel reduced recoil energy of the cartridge, the bolt was reduced in weight by about 15% and the recoil spring shorted just slightly as well, to ensure proper cycling. The result was formally designated the Sten MkII(S). It retained the selective fire capability of the Sten, but was not to be used in automatic mode, as doing so could compress the baffles together and damage them. In total, 5,776 of these silent submachine guns were made. The design was followed by a more sophisticated Sten Mk6 (essentially a silenced Mk5), but remained in active use with the British military into the early 1970s.
Finding completely original and intact examples of the MkII(S) is extremely difficult today, and this one is a rare privilege to examine!