Sten is the simplest mass-production submachine gun used during World War 2. It evolved form a more complex and expensive Lanchester. Sten Mk.2 (and later Mk.3) was simple to the extreme. All-metal and made to generous tolerances it was much faster and cheaper to make than an Enfield rifle. It cost $6 to make and most parts were simple metal pipes.

While its sights were rudimentary, the low recoil of its 9mm round and slow rate of fire enabled even poorly trained users to employ it effectively. So cheap it was, that the Germans produced their own version starting in 1944. The American equivalent was the M3 "Grease gun", a similarly crude and cheap submachine gun that largely supplanted the complex and expensive Thompson.

Consider how few parts make up this gun. Remember that the chamber and barrel is the only parts that require good steel. Almost every metal-working shop is set up to make parts for submachine guns - and assembly can happen elsewhere. If guns are severely restricted in some country or another, producing "lead hoses" like these may well be easier than hunting armed officials for their guns.

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Mk.5 was the "deluxe" model, complete with rifle rights and a pistol grip. The current British Sterling SMG evolved from this ugly contraption.

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