Smith & Wesson Model 1940 Light Rifle
In 1939 the British gov't asked Smith & Wesson to design a 9mm light rifle for the British military. The design was developed in the US using commercial 9mm Luger ammo. The more powerful British 9mm service ammo broke several receivers within 1000 rounds of testing. The contract was cancelled after 60 prototypes, 750 Mark Is, and 200 Mark IIs with strengthened receivers. Of 1010 known examples, few were ever issued for military service. The gun apparently did not get past the proving ground test stage. At the end of the war, five examples were placed in UK military museums and the rest were destroyed or dumped at sea.
The buttstock was Tennite plastic, the gun fired semi-auto-only (one shot per trigger pull) even though it generally resembled a submachinegun. The forward grip was metal and housed the magazine and the cartridge ejection chute. The enclosed downward ejection system kept out dirt, but made clearing a jam difficult.
Model 1940 Light Rifle
Type: Semi-Automatic Carbine
Place of origin: United States
Production history:
Designer: Edward S. Pomeroy
Manufacturer: Smith & Wesson
Specifications:
Weight: 9 pounds (4 kg)
Length: 32 inches (81 cm)
Barrel length: 9.75 inches (25 cm)
Cartridge: 9×19mm Parabellum
Action: Blowback, Open Bolt
Feed system: 20-round box magazine
At 9 pounds it was pretty heavy for a "light rifle".
"A Bad Investment: S&W Model 1940 Light Rifle"
by Bruce Canfield - American Rifleman, Wednesday, October 17, 2012
https://www.americanrifleman.org/articles/2012/10/17/a-bad-investment-sw-model-1940-light-rifle/
US legality: ATF Curio & Relics List lists the "Smith & Wesson, model 40, Light Rifle, caliber 9mm parabellum" as a Curio & Relic, more likely to be valued as a collectible than as a weapon. NFA rules apply: they are still as Short Barrel Rifles (SBS) and open-bolt semi-auto theoretically convertible to full-auto.
https://www.atf.gov/firearms/docs/g...r-relics-list-1972-2007-atf-p-530011/download