MANUFACTURER – Royal Small Arms Factory, Enfield, London, England. Royal Ordnance Factory, Theale, England. Royal Ordnance Factory, Fazakerley, Liverpool.
STATUS – Obsolescent
SERVICE – British Military and Commonwealth, Special Operations Forces, Intelligence (Special Operations Executive – SOE), Resistance forces and Allies.
In November, 1942, 100 special Sten guns were tested by the Ordnance Board in secret trials. The Stens had been fitted with six different styles of sound suppressors. The development and testing of a suppressed or “silenced” Sten gun were conducted at a request from the European Resistance forces inside of Nazi-held Europe and the British Special Operations community. An additional test of more suppressed Sten designs was conducted in February, 1943 and field tests done with the weapons in the hands of troops in January and February 1944. From all of those tests and trials, a design was chosen for production.
The design of the Sten Mk II made it an excellent subject for suppression. The barrel assembly could be easily unscrewed and a suppressor assembly put in its place. With the location of the front sight being on the front of the receiver, there was no interference with the sight line when the suppressor was mounted. The new weapon was designated the Carbine, Machine, Sten, 9mm Mk2 (S), the “S” standing for “Special Purpose” rather than the widely believed “Suppressed” or “Silenced.”
The suppressor itself consisted of a small expansion chamber with an internal free volume of 44.3 cc (2.7 cubic inches). This chamber surrounds a shortened barrel with two rings of 6 (12 in total) 2.4 mm (0.093 in) gas bleed holes drilled through the barrel. The first ring is just in front of the chamber of the barrel with the second being 16mm (0.63 in) back from the muzzle. These bleed holes combined with the short barrel help to lower the muzzle velocity of standard Mk2z ammunition to below the speed of sound. The low muzzle velocity eliminates the sonic “crack” of a supersonic projectile travelling away from the weapon while the design allows for standard-issue ammunition to be used.
The propellant gases, after expansion and some cooling in the expansion chamber, pass back through the barrel into the front tube of the suppressor, which holds the conical baffle stack. The front tube has an internal free volume of 213 cc (13.0 cubic inches) and holds a stack of 18 spaced conical baffles, the cone of the baffles each having an 11.2 mm (0.44 in) hole in their center and facing back to the breech of the weapon. The six front baffles have half of the cone cut away and are stacked alternating with the open side of the cone opposite of each other. The baffle stack helps break up the flow of the propellant gases, cooling them prior to their exiting the suppressor. Additionally, there is a stack of three felt baffles (packing pieces) and an insulating fiber washer held in a tight stack at the muzzle of the suppressor against a 19th baffle, this one a simple thick flat steel disk. The felt baffles each have a 7.6 mm (0.30 in) holes in their center. These holes enlarge through the use of the suppressor and the felt (flexible) baffles need replacing on a regular schedule for maximum suppressor efficiency.
The gas bleed holes in the barrel allows gas into the expansion chamber to lower the muzzle velocity to less than the speed of sound. But the gas bleed holes also cut back on the pressure available to push the cartridge case back against the bolt, sometimes to the point of the bolt not going back far enough to engage the sear, but being able to pick up a round from the magazine, chamber and fire it. This results in a runaway gun (uncontrolled automatic firing). To help prevent these problems, the MkII(S) weapon has a lightened breech block weighing 493 g (17.39 oz) as compared to a standard Mk II Sten breechblock with a weight 597 g (21.06 oz) along with a shortened (14 coil) mainspring as compared to the standard (15.5 coil) of the Mk II mainspring. These changes allow for a greater functional reliability of the Mk II(S) when it is fired. All suppressed Sten guns were carefully test fired before leaving the factory and the bolts were engraved with the serial number of the weapon that had been fitted to.
A last part of the suppressor was an insulating thermal sleeve that surrounded the rear of the body where the operator’s hand would hold. This thermal sleeve was very important as the suppressor would very quickly heat up as it was fired. The body of the suppressor was wrapped with asbestos string for about 15 cm (6 in). Over the string was wrapped a canvas cover with eyelets along both sides and a horsehair liner. The eyelets allowed the thermal sleeve to be laced up and strapped down tightly to the suppressor.
The MkII(S) weapon was very successful in service, even though it was declared obsolescent in April, 1945 in the same order that it was officially introduced. In spite of this very odd situation (possibly brought about for security reasons) the Mk II (S) remained in service with the British Military, Commands, and others until the early 1970s.
