Soviet World War Two 50mm Light Mortars (RM-39 & RM-40)

Published on August 25, 2025
Duration: 13:01

This video provides an expert-level overview of Soviet 50mm light mortars, specifically the RM-39 and RM-40, presented by Ian McCollum of Forgotten Weapons. It details their historical development, technical specifications including a unique gas-venting range adjustment system, and mechanical features. The video also covers their comparison to German counterparts and their post-war surplus history, including Finnish capture and US importation.

Quick Summary

The Soviet RM-39 and RM-40 50mm light mortars featured a unique gas-venting system for range adjustment, allowing variable settings without changing the firing angle. The RM-39 was complex, while the RM-40 simplified design for wartime production. Many surplus units in the US originated from Finnish capture and later Interarms importation, often classified as NFA Destructive Devices.

Chapters

  1. 00:00Introduction: Soviet 50mm Mortars
  2. 01:10Comparison & Range Adjustment System
  3. 03:16The RM-39 Pattern Explained
  4. 04:53RM-39 Mechanical Features
  5. 07:06The RM-40 Pattern: Wartime Simplification
  6. 09:21RM-40 Internals & Baseplate
  7. 10:58Surplus & Capture History

Frequently Asked Questions

What was the primary reason for the rapid development of Soviet 50mm light mortars like the RM-39 and RM-40?

The Soviets rapidly developed models like the RM-38, RM-39, RM-40, and RM-41 between 1937 and 1943 in an effort to simplify complex designs and improve battlefield effectiveness, though they were eventually replaced by larger caliber mortars.

How did the Soviet 50mm mortars adjust their range?

A unique feature was the range adjustment system using propellant gas venting. Holes at the bottom of the mortar tube released gases, allowing the range to be controlled without altering the fixed firing angle, enabling settings up to 800 meters.

What is the significance of the RM-40 compared to the RM-39?

The RM-40 represents a move towards wartime simplification. While retaining key features like gas venting, it uses stamped sheet metal for bipod legs and simpler mechanisms, speeding up production compared to the more complex RM-39.

Where did many of the surplus Soviet 50mm mortars in the US originate?

Many were captured by Finland during the Winter War and Continuation Wars. In the 1960s, Interarms purchased over a thousand units from Finland and imported them into the United States.

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