Designed in 1936 by Melvin Johnson, the M1941 Johnson Automatic Rifle was a competitor to the M1 Garand, but not introduced in time to actually be adopted din place of the Garand. Instead, Johnson hoped to have his rifle accepted as a parallel second option for the US military in case something went wrong with the rollout of the Garand, or production simply couldn’t meet the required levels.
However, Johnson was not able to make his case to the military successfully. A small number of Johnson Light Machine Guns were acquired by the US Paramarines and the First Special Service Force, and a large order (30,000 rifles) was placed by the Dutch government for shipment to the colonies in southeast Asia (it is from this order that the M1941 designation comes). However, those colonies fell to the Japanese before a significant number of rifles were able to be shipped out. This left a substantial number of rifles orphaned in the US, and a small number of these were unofficially put in service by acquisitive Marines, mostly in the Pacific theater.
Mechanically, the Johnson is a short recoil system with a rotating bolt (very similar to the later AR-15 bolt, which Johnson would influence). It is chambered for the standard .30-06 cartridge, and feeds from a 10-round rotary fixed magazine which can be fed by stripper clips or with individual cartridges.