RM-38 was a Soviet 50mm light infantry mortar, developed as a variant of the M1938 120mm mortar. The barrel was clamped at two elevation angles only - 45 and 75 degrees. Range variations were made by altering a sleeve round the base of the barrel. This sleeve opened a series of gas ports which bled off exhaust gases and so determined the range.
The project was deemed overly complex and expensive, and was only produced for a short time, before being replaced by the Model 1939. Despite the small number produced, some fell into Germand hands in 1941, who introduced them as the 5cm Granatwerfer 205/1 (r).
The? RM-38 or 50-RM-38 was based on the Stokes mortar. It was further developed as the RM-39 and RM-40.
The Red Army of the USSR divided mortars into company (RM) battalion (BM) and regimental (HM) mortars. Development of a light 50mm company mortar started in 1937. ? The RM-38 was approved for use in 1938 and entered production in 1939. In the space of just over a year RM-39, RM-40, and RM-41 replaced each other in succession. RM-41 remained in production until 1943, when the USSR decided to cease making 50mm mortars. Only RM-41 was a new design, the others being incremental improvements of the original RM-38.
The problem of only having two fixed elevations and thus needing to adjust the range with the complex adjustment of gas escape made for inaccurate ranging and was dangerous to the mortarman as well. The minimum shooting range of 200m was felt to be impractical in Red Army use as well.
RM-39 added a protective shield which directed the escaping hot gases away from the operator.
Barrels got shorter with each new model.
Essentially a new design influenced by German 50mm mortars, which continued in use until 50mm mortars were removed from Soviet Army service. ? This mortar was without a tripod but instead relied on its barrel yoke which contained traverse and elevation adjustments. ? The gases now veted under the muzzle via a tube.
All models saw widespread use by the USSR in WWII. ? Captured in large numbers they were also re-used by the Finns and Germans. After WWII the USSR supplied them to North Korea and Vietnam.
The Finns were apparently not impressed with these Russian 50mm mortars.
The Finns found the RM-39 relatively accurate in use and setting the mortar ready to fire took only about one minute. The mortar was no substitute for the 80-82mm mortars however, perhaps due to only having 100g of TNT in the shell, less than some hand grenades.