In 1944, the British General Staff issued a specification for a new submachine gun. It stated that the weapon should weigh no more than six pounds (2.7 kg), should fire 9×19mm Parabellum ammunition, have a rate of fire of no more than 500 rounds per minute and be sufficiently accurate to allow five consecutive shots (fired in semi-automatic mode) to be placed inside a one-foot-square target at a distance of 100 yd (91 m).
To meet the new requirement, George William Patchett, the chief designer at the Sterling Armaments Company of Dagenham, submitted a sample weapon of new design in early 1944. The first Patchett prototype gun was similar to the Sten insofar as its cocking handle (and the slot it moved back and forth in) was placed in line of sight with the ejection port though it was redesigned soon afterwards and moved up to a slightly offset position. The army quickly recognised the Patchett's potential (i.e. significantly increased accuracy and reliability when compared with the Sten) and ordered 120 examples for trials. Towards the end of the Second World War, some of these trial samples were used in combat by airborne troops during the battle of Arnhem and by special forces at other locations in Northern Europe where it was officially known as the Patchett Machine Carbine Mk 1. For example, a Patchett submachine gun (serial numbered 078 and now held by the Imperial War Museum), was carried in action by Colonel Robert W.P. Dawson while he was Commanding Officer of No. 4 Commando, during the attack on Walcheren as part of Operation Infatuate in November 1944. Because the Patchett/Sterling can use straight Sten submachine gun magazines as well as the curved Sterling design, there were no interoperability problems.
After the war, with large numbers of Sten guns in the inventory, there was little interest in replacing them with a superior design. However, in 1947, a competitive trial between the Patchett, an Enfield design, a new BSA design and an experimental Australian design was held, with the Sten for comparison. The trial was inconclusive but was followed by further development and more trials. Eventually, the Patchett design won and the decision was made in 1951 for the British Army to adopt it. It started to replace the Sten in 1953 as the "Sub-Machine Gun L2A1". Its last non-suppressed variation was the L2A3 but the model changes were minimal throughout its development life.
The Sterling submachine gun is constructed entirely of steel and plastic and has a shoulder stock, which folds underneath the weapon. There is an adjustable rear-sight, which can be flipped between 100 and 200 yard settings. Although of conventional blowback design firing from an open bolt, there are some unusual features: for example, the bolt has helical grooves cut into the surface to remove dirt and fouling from the inside of the receiver to increase reliability. There are two concentric recoil springs which cycle the bolt, as opposed to the single spring arrangement used by many other SMG designs. This double-spring arrangement significantly reduces "bolt-bounce" when cartridges are chambered, resulting in better obturation, smoother recoil and increased accuracy. Additionally, the Sterling uses a much-improved (over the Sten) 34-round curved double-column feed box magazine, which is inserted into the left side of the receiver. The magazine follower, which pushes the cartridges into the feed port, is equipped with rollers to reduce friction. The bolt feeds ammunition alternately from the top and bottom of the magazine lips, and its fixed firing pin is designed so that it does not line up with the primer in the cartridge until the cartridge has entered the chamber.
The Sterling employs a degree of what is known as Advanced Primer Ignition, in that the cartridge is fired while the bolt is still moving forward, a fraction of a second before the round is fully chambered. The firing of the round thus not only sends the bullet flying down the barrel but simultaneously resists the forwards movement of the bolt. By this means it is possible to employ a lighter bolt than if the cartridge was fired after the bolt had already stopped, as in simple blowback, since the energy of the expanding gases would then only have to overcome the bolt's static inertia (plus spring resistance) to push it backwards again and cycle the weapon; whereas in this arrangement it fights its forwards momentum as well, and thus some of it is used up. The lighter bolt makes not only for a lighter gun, but a more controllable one since there is less mass moving to and fro within it as it fires.
The suppressed version of the Sterling (L34A1/Mk.5) was developed for covert operations. This version uses a ported barrel surrounded by a cylinder with expansion chambers to reduce the velocity of the bullet so that it doesn't break the sound barrier and thus cause a sonic boom, along with decreasing the muzzle blast and flash. This is so effective that the only sounds during firing are from the bolt reciprocating and the barely audible explosive discharge. The Australian and New Zealand SAS regiments used the suppressed version of the Sterling during the Vietnam War. It is notable for having been used by both Argentinian and British Special Forces during the Falklands War. A Sterling was used by Libyan agents to kill WPC Yvonne Fletcher outside the Libyan Embassy in London, which sparked the 1984 siege of the building.