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Designed in 1939 by S&W engineer Edward Pomeroy, the S&W Light Rifle is an extremely well manufactured but rather poorly thought out carbine. It is a 9mm Parabellum open-bolt, semiautomatic, blowback carbine feeding from 20-round magazines. It was tested by the US military at Aberdeen Proving Grounds, and rejected for a number of reasons, including not being in the US standard .45 ACP cartridge and not being full automatic. However, the British were in dire need of small arms, and S&W decided to pursue sales to the UK rather than redesign the gun to US taste.
The UK ordered a large number, but upon putting the first guns through trials found them to be unsatisfactory. The 9x19mm loading used by the British was substantially hotter than what S&W had used in designing the weapon, and receiver endocarps were shearing off in as few as 1000 rounds under British testing. The British cancelled the order, and took delivery of S&W revolvers in lieu of a refund on their (sizable) down payment. At the end of the war, all but 5 of the 1,010 guns delivered were destroyed.
In 1974, crates of leftover Light Rifles were discovered in the basement of S&W - 137 MkI types and 80 MkII types. These were sold as a batch to Bill Orr of GT Distributors, who then sold them on the commercial market. Orr also petitioned ATF to exempt the guns from NFA short-barreled rifle classification (the guns have 9.75” barrels), and was successful - so these transfer as ordinary rifles despite their short barrels.
The difference between the MkI and MkII is the safety and the firing pin. The MkI has a lever safety which locks the bolt in the rearward position, and a floating firing pin with a lever actuator like a Beretta M38. The MkII has a rotary sleeve safety which locks the bolt either forward or rearward (a better system), and a fixed firing pin milled into the bolt face. Note that contrary to most literature, the MkII receiver was not strengthened to alleviate the durability problems found by British testing.