The first records of Patchett's submachine gun date back to 1942, where it was tested at RSAF Enfield. The initial prototype was described in official reports as "essentially, with the exception of the trigger mechanism, a Lanchester without butt or sights". The Ordnance Board advised Mr. Patchett to improve the design and resubmit it. The improved model, named the Mk.I, appeared in February 1943. About 110 models were made and issued in very limited numbers to the 6th Airborne Division in Normandy and the 1st Airborne Division in Arnhem.
Suggestions were made to improve the design and in 1944 a newer model with a ribbed bolt was introduced. It was named the Patchett Mk.II and was tested against the BSA machine-carbine, the MCEM-3, and the Australian MCEM-1 in 1947. It fired at about 600 rounds per minute but the trigger mechanism was slightly faulty and caused some firers to receive bad bruises on their fingers. This led to a redesign of the trigger mechanism, which ended up increasing the rate of fire well over 600 rounds per minute, which was considered excessive. BSA's Machine Carbine was favoured in the 1947 trials.
In May 1951, trials were held once again, this time with the BSA machine-carbine, the Madsen Model 50 and the Australian MCEM-2. It was clear during these trials that the Patchett gun was the best weapon, as BSA's weapon suffered severe cocking issues, the Madsen's magazine could not function properly in mud and sand, and the Australian MCEM-2 had a complete failure to eject at one point. The Patchett gun's rate of fire had been reduced using a "net effect" achieved through replacing the secondary return spring. The Patchett gun excelled in the field stripping tests, as no tools were required to strip the weapon.
After the trials, the Ordnance Board suggested that the British Army adopt the Patchett gun if the EM-2 rifle failed to perform well in the submachine gun role. The Madsen was recommended for Line of Communication troops if the EM-2 turned out to be successful in the SMG role. In the end, troops were never issued the EM-2 at all, so the Patchett was adopted as the "Submachine gun, L2A1" on the 18th of September 1953.
Despite the weapon being officially named the "Patchett Machine Carbine" during trials, and even after adoption, troops simply referred to the weapon as the "Sterling". The name stuck and it became Sterling Armament Company's most iconic weapon. After the 50's, it was almost never referred to as the "Patchett gun".