The Military Carbine, Experimental Models were a series of experimental submachine guns developed at RSAF Enfield during the 1940s. The aim of the project was to create a suitable replacement for the Sten in British Army service. Six MCEM guns were made in total, but none of the designs were successful and the project was abandoned in 1947.
RSAF Enfield began work on the MCEM project around 1942, and aimed to have finished prototypes ready by the end of the war; the British Army had no intention of keeping the Sten gun in service once the war was over and sought an immediate replacement. Enfield divided its design teams up by nationality and had two separate teams working on the MCEM project: one consisting of British engineers and another consisting of Polish engineers. The British team was headed by Harold J. Turpin, co-designer of the Sten. The Polish team was headed by Lt. Jerzy Podsędkowski.
The British team's first prototype was known as the MCEM-1. The basic action and trigger mechanism were taken directly from the Sten. A new addition was the implementation of a spring collar around the return spring that would grip the bolt every time it traveled back, causing a friction delay that moderated the fire rate.
Externally the gun had an unusual build. The wooden stock slotted into the tubular pistol grip and could be detached. The cocking handle was built with a dust cover to avoid dirt and grime from entering the cocking slot. The magazines used for the MCEM-1 were also unconventional; they consisted of two 20-round box magazines welded together side-by-side to create a double magazine. To facilitate for this, the magazine feed was built as a horizontal guide into which the magazine slid across.
The MCEM-1 was briefly loaned out to the Royal Marines Small Arms School in Gosport and tested there, where it was reported to function satisfactorily, with some minor stoppages. It was then subjected to internal tests at RSAF Enfield against the MCEM-2, but both weapons were considered unsuitable in their current form and were sent back to their respective designers for improvements.
The MCEM-3 was simply a development of the MCEM-1, designed to conform better to the Ordnance Board's strict specifications. The internal action was the same but the spring collar that caused the friction delay was removed. The barrel was modified with bayonet fittings and the double magazines of the MCEM-1 was replaced by a curved 20-round magazine, which could be loaded by 5-round clips. Slight modifications were made to the design of the grip and stock.
The MCEM-2, MCEM-3, and MCEM-6 were selected for further internal tests at Enfield in 1946, with the MCEM-3 eventually being selected as the design most worthy of submission to military trials. The other designs were rejected and from June 1947 to April 1948, the MCEM-3 was trialed against various other submachine guns, such as the Patchett, BSA, and Kokoda. Numerous faults were noted, such as the weapon's tendency to overheat after sustained fire. The Ordnance Board did not think the MCEM-3 was up to the standards of the other weapons offered and it was eliminated during the early stages of the trials.
The Polish team's first prototype, completed after the MCEM-1, was known as the MCEM-2. It was a completely original design built from scratch, and was one of the first submachine guns to feed its magazines through the pistol grip. The long cylindrical bolt enveloped the barrel and chamber in a "wrap-round" design. All but one inch of the bolt was hollowed-out, and the firing pin was located inside. Compacted between the bolt and the return spring was a spring-loaded ejector device which, upon the bolt's return, would force itself through the bolt face and push the spent casing out an opening in the rear end of the bolt, where it would be released through the ejection port.
The MCEM-2 was of a compact design and could be classified as a machine pistol. It was only 14 inches long and featured a detachable wireframe stock covered in a canvas fitting that could double as a holster. The magazine held only 18 rounds and the fire rate was around 1000 rounds per minute, meaning the magazine would be expended very quickly during automatic fire.
Internal tests were arranged to compare the MCEM-1 and MCEM-2. Neither weapon was favored and both were sent back to the design departments for redesign, so as to comply to the Ordnance Board's specifications.
The MCEM-4 was developed around 1943. The designer was Lt. Wikter Kulikowski, an SOE officer who had developed the suppressor modification for the Sten Mk.IIS. The MCEM-4 was tested only once, against the Mk.IIS, indicating that it may have also been a suppressed weapon. It is thought to have been a suppressed version of the MCEM-2 but there are no documents to confirm this.
The MCEM-6 was the result of improvements made to the MCEM-2. It retained the same basic design but was built with a heavier bolt to moderate the fire rate and an extended barrel with bayonet fittings. As a result of the modifications, accuracy was better and the fire rate was reduced to a more controllable 600 rounds per minute. The trigger mechanism was also modified with a flywheel-type delayer housed forward of the trigger guard, similar to the type seen on the BSA-Király submachine gun. These modifications were performed by Captain Aleksander Ihnatowicz-Świat.
The MCEM-6 competed against the MCEM-2 and MCEM-3 during the final internal tests. Enfield decided that the MCEM-3 ultimately had the best chance at military adoption and was worth developing further. As such, the MCEM-6 was rejected and the funding for it was pulled. It never reached military trials.
Sometime between the development of the MCEM-4 and the MCEM-6, an MCEM-5 was developed. It may have been related to the SPARC, but since no records of the gun have survived, it is not known how the weapon functioned, what it looked like, or if it was ever tested.