MANUFACTURER – Lines Brothers Limited, Merton Works, South-West London, England
STATUS – Obsolete
SERVICE – British Military, Home Guard, and Resistance forces.
The Sten design was developed so that none of the parts were of such precision that they couldn’t be produced by subcontractors. These were companies that produced only a few different parts which would be forwarded to one of the assembly facilities for complete construction. The contracting of the parts was according to what the company facilities could produce, no matter what they had been making prior to World war II. The Lines Bros, Ltd, Merton Works of SW London had been a toy and bicycle manufacturer since the end of World War I. They stamped and formed metal with several decades of experience behind them. The company received contracts for components for a number of weapons including rifles, machineguns, Lanchesters, Mark I and II Sten guns as well as magazines. The fabrication engineers at Lines Bros carefully examined the Mk I Sten and suggested they could produce an improved, and simplified, version completely within their own plant from steel pressings, stampings, and welding with minimal machining. And they would require very little in the way of materials except for sheet steel.
Prototypes of the Lines Bros design were tested during the Winter of 1941-42. Flaws were found in the design, but these could be readily remedied. The officers of the company admitted that they had little experience in producing guns, but they had a lot of knowledge in forming sheet metal.
The design that became the Mk III Sten was produced primarily out of sheet metal with the exception of the barrel, bolt, bolt handle, and springs. Parts were riveted and welded together, including the receiver tube. The main body of the weapon was stamped and formed from sheet metal. Internal components (the barrel and its bushings) placed inside, and fixed in place with soldering and rivets. Then the body was finally formed and welded into a tube. The weld ran along the entire top of the weapon and was a notable feature in the design. The magazine well was formed of sheet metal and welded to the left side of the receiver. On the right side of the receiver a guard was riveted in front of the ejection port to help prevent an operator’s fingers from entering the port while firing. The body had a number of small vent holes drilled into it which would allow air to circulate a bit around the barrel. They could also be used as sling mounts.
This was the single most common Sten design produced besides the Mk II. The Mk III actually was a step backwards in a way from the Mk II back to the Mk I* design in that the body was a single part that included the receiver and barrel shroud with only a very short portion of the barrel muzzle exposed. The rear of the receiver would accept any of the Sten buttstock designs, though the weapon was most often seen with the Mk II “T” buttstock.
There had been sheet steel Mark II and some Mark I Sten guns produced early in the designs manufacture. There had been a shortage of steel tubing of the proper dimensions so sheet metal was formed around a mandrel with the two edges meeting at the bottom of the receiver where they were welded together. The rear portion of the sheet metal tube was extended down to make up the side walls of the trigger housing. These weapons had a number of failures in the field and, by July, 1943, all were recalled to be utilized as spare parts at the most. But the Lines Brothers had long experience in production tubing formed from sheet and they design for the Sten had no such problems.
The Mark III design was very plain and as simplified as possible. It was not a high-quality weapon, but it worked and it worked well. Plus, it could be made in quantity. While multiple manufacturers were turning out Sten Mark II weapons in numbers, a monthly output could reach 110,000 pieces. Only the Lines Bros plant was turning out Mk III Stens, at a rate of 70,000 per month! The production of the Mk III weapon was suspended in late 1943 for production to concentrate on the Mk II design after Lines Bros and the Merton facility had produced over 1 million Mk III guns.
