The Robinson S.R. Model 11 9mm Machine Pistol, also known as the SR-11 or Robbie gun, was an Australian machine pistol designed by Russel Robinson in 1943 and produced by the Lithgow Small Arms Factory in prototype form only.
The SR-11 was designed by Russel Robinson, an Australian designer who was known for his work with constant recoil arms, on advice of the Australian Army Inventions Directorate to develop a 9mm version of the Gatenby gun they had examined prior; Robinson himself had examined the Gatenby and was impressed by various aspects of the design, which led him to design the S.R. Models 8 and 10. These models were then redesigned to become what would become known as the SR-11.
Robinson was instructed by then-Director of Artillery Eric Hall to design the pistol and laid down some 13 specifications for him to meet; after some revisions the amount of specifications was then increased to 24. Robinson designed the weapon beginning in 1943; he finished the drawings for the SR-11 in March 1944 and approval was granted in November the same year for the construction of the weapon; however, heavy modifications to the design were required and the design was only completed by February 1945, with newer and heavier slides being manufactured in England for this purpose. Major parts for the SR-11 had been fabricated by September 1945, with the prototype having fired some 1,600 rounds in testing. A total of four weapons were eventually produced.
The weapon was then trialed in front of the United Kingdom's Ordnance Board in tests run by Major Hall in 1946; the weapon was noted as being significantly more accurate than the STEN Mk V and the MCEM-2 in fully automatic fire, but not as accurate when fired semi-automatically. Sand and mud tests also exposed a number of other problems with the weapon's systems.
As Robinson's work at the time focused more on machine guns for the British, the SR-11 was only given spasmodic attention and little work was done to improve the design. However, after those initial tests, the Ordnance Board felt that if the weapon felt like a pistol and looked like a pistol, it should at least shoot like a pistol; as such, they advised Robinson to redesign the SR-11 to be accurate in semi-automatic fire while also retaining its accuracy in fully automatic fire. This redesign would be known as the S.R. Model 16, however the weapon was never built apart from some firing fixtures.
In an attempt to increase the weight of the slide of the SR-11 while attempting to not make the weapon bulkier, Robinson experimented with an alloy made by General Electric known as the Heavy Alloy and manufactured new slides for the SR-11, which were slightly longer and thicker. He made more modifications to the design as well, with his idea that further tests would lead to the weapon needing another redesign and further manufacture; none of these tests ever occurred however, as by then Robinson had moved to the United States and as the British had adopted the Patchett gun by then, there was no need for a new automatic weapon and all development ceased.
Robinson patented a version of his SR-11's action as late as 1970, though no success was found with this version of the action. Serial number 3 is currently located at the National Firearms Center in Leeds, as part of the Royal Armouries' research collection.
The SR-11 is a blowback-operated machine pistol which fires from an open slide. The weapon features a tubular buttstock which is screwed onto the extended tang of the weapon. Controls are ambidextrous. While the pistol may appear to look like a normal pistol at first glance, it has one defining feature; a helical rotating floating barrel.
The floating barrel allowed the weapon to pre-absorb most of the recoil with the forward movement of the slide. The weapon also has an automatic magazine ejection system; when the follower strikes the magazine retainer, the retainer is released, allowing the magazine ejector to throw the magazine out of the magwell and out the gun. As the magazine is thrown out, a heel on the ejector rises; when a magazine is loaded, the rear of the magazine engages this heel, rocking the ejector upwards. As the ejector is rocked upwards, the slide is allowed to move rearwards by about 0.25 inches (0.64 centimetres) to the cocked position, allowing the weapon to fire.
As the trigger is pulled, a precompressed driving spring located in the slide forces it forward, with the rear end bearing rearward against the barrel and holding it against the rear stop. As the slide moves forward, it forces the barrel to rotate through a series of helical grooves cut into the barrel at a fairly constant speed of about 1,200 rpm. As the slide slams forward, the rotational speed of the barrel is about half that of the bullet; the fixed firing pin then pierces the primer of the loaded bullet and fires the round, although as combustion develops a slight delay occurs as the slide wants to move forward before recoil is sufficient enough to arrest and force the slide back. To pre-absorb the recoil, the barrel also slides forward with the slide.
As the bullet leaves the muzzle, the barrel rotation is arrested and reversed by the rotating bullet, theoretically reducing recoil; the reverse happens when the slide slams back rearward. A small blow can be felt when the slide hits the rearward position. Due to this action, the pistol is not considered a true constant recoil weapon.
- The World's Submachine Guns, Nelson & Lockhoven, 1963