The Soviet 50mm light infantry mortar of World War 2 (1939-1945) evolved along a singular line beginning with the 50-RM 38 model of 1938 which, itself, was influenced by the British Stokes 3-inch mortar design. While serviceable, the system relied on an inefficient manual gas management system to define precise engagement ranges and the 50mm projectile was limited in its reach and shock value. While the projectile was left unaltered, the RM-38 was developed into the slightly improved model of 1939 as the 50-RM 39 / RM-39. The RM-39 showcased a barrel length of 775mm and a weight between 14- and 17-kilograms depending on production. The original RM-38 measured with a 780mm barrel and a 12-kilogram weight.
In 1940 there proved yet another evolution in the 50mm light mortar line and this became the 50-RM 40 (M1940 or "Model 1940"). The weapon was consistent with previous offerings in being conventional in its form and function. There was a launch tube, baseplate and bipod comprising the entire system with included optics, as crude as they were. The bipod was manufactured of pressed steel. The basic accepted operation was retained - at least two crew, one to manage the launch tube and aiming and the other to feed projectiles into the muzzle. An awaiting firing pin ignited the propellant of each charge and sent it along its defined path.
As with the RM-38, the RM-40 proved no better in practice. The 50mm was still a limited projectile and the RM-40 held a limited range. It was, perhaps for the best, that the Soviet attention began turning to larger-caliber instruments with siege mentality in mind such as the massive 160mm mortar system of 1943. To that end, stocks of RM-40 and similar light mortars were used when available and in some number. As such, captured stocks then fell to the Germans who pressed them into action against their former owners under the army designation of 5cm Granatwerfer 205(r) - "r" to indicate their Russian origins.
As the RM-40 superseded the RM-39, it was itself surpassed by the all-new RM-41. Instead of it being based on the RM-38 and, therefore the British Stokes design, it was developed from study of captured German 50mm mortars.