152 mm mortar M1931 ( NM ) 152 mm mortar M1931 ( NM ) ( ) was a 152 . 4 mm ( 6 inch ) artillery piece originally developed by the German arms manufacturer Rheinmetall . The gun was produced in limited numbers in the Soviet Union and saw action with the Soviet Army in World War II . A modified version of the design was also adopted in Germany , as 15 cm sIG 33 . Description . The gun had monobloc barrel fitted with horizontal sliding breech . The recoil system consisted of hydraulic compressor and hydropneumatic recuperator , both mounted in the gun cradle . The recoil length was fixed . The carriage was of box trail type , with equilibrator , suspension and a folding spade . Metal wheels had removable rubber tires . The gun was equipped with a shield . Development and production . The first 152 mm divisional mortar for the RKKA was developed in by Kirov Plant in 1930 . The gun weighed about 1 , 500 kg . It utilized a breechblock and other elements of the 152 mm fortress howitzer M1909 . It is not clear whether the piece was ever completed . From late 1920s , USSR looked for foreign assistance in modernizing its artillery . Germany could and was willing to offer such assistance . The cooperation with USSR allowed Germany , constrained by the Treaty of Versailles , an opportunity to proceed with arms development . In 1929 , German arms manufacturer Rheinmetall created a dummy company " Butast " for contacts with USSR . In accordance with the Sovnarkom decision from 8 August 1930 , on 28 August in Berlin a secret agreement was signed . Germans undertook to help USSR with production of six artillery systems : For $ 1 , 125 mil . Rheinmetall supplied pre - production samples , documentation and parts from which in USSR a few pieces of each type could be assembled . Among other pieces , Rheinmetall brought to USSR eight 152 mm mortars . The guns went through ground trials in June 1931 and were tested by the army in August 1932 . The weapon was adopted as 152 mm mortar M1931 . In the contemporary documents it is often referred to as N or NM . The gun was in production from 1932 to 1935 at Plant no . 172 ( " MZM " - Motovilikha Machinery Plant ; ) . The production continuously lagged behind the schedule . In 1932 , 5 mortars were manufactured ; in 1933 - 50 , in 1934 – 59 , and in 1935 – 15 pieces . During the production period the gun was repeatedly modified ; for example , the barrel of was lengthened by 65 mm . In 1937 a modernized variant was developed . This variant , designated ML - 21 , reached factory trials on 27 March 1937 and ground and army trials in 1938 . The trials revealed some minor defects . The ML - 21 was never adopted . Meanwhile , in Germany a modified variant of the original Rheinmetall design was adopted as 15 cm sIG 33 . Self - propelled mounts . The NM was experimentally mounted on T - 26 tank chassis . The resulting self - propelled gun , designated SU - 5 - 3 , successfully underwent factory trials in 1934 and even took part in a parade at the Red Square . However , in 1935 the project was cancelled , because the T - 26 chassis was considered too weak for a 152 mm piece . The fate of the prototype vehicle is unknown . There is an unconfirmed information that it was rebuilt as SU - 5 - 2 , armed with 122 mm howitzer M1910 / 30 . Employment . The NM was employed in divisional artillery , which from 1935 to June 1941 included a battalion of 152 mm howitzers . On 1 November 1936 RKKA possessed 104 pieces ( including three practical and one non - operational ) . By June 1941 only 51 of them remained and , according to the historian M . Svirin , all these were in mobilization reserve . Some pieces saw combat early in the German - Soviet War . In 1942 , new ballistic tables for the mortar were released . Summary . Despite some advantages of the NM , notably its versatility and very light weight for a 152 - mm piece , the production was cancelled after a limited number of guns were built . According to M . Svirin , complexity of the design was the main reason . For example , Soviet ordnance plants experienced major problems with production of sliding breechblocks . Also , the NM poorly fitted the Soviet artillery doctrine , mainly because of its range - too short for a divisional gun . The related German design - the 15 cm sIG 33 - was employed as a regimental weapon ; it was somewhat heavy for a regimental piece . In the RKKA , the niche of heavy regimental weapon was filled by a 120 mm infantry mortar , a cheaper , lighter weapon with slightly longer range , but on the other hand less powerful , less accurate and less versatile . Eventually , a need for a divisional weapon more powerful than a 122 mm howitzer became apparent ; in 1943 the RKKA adopted another infantry mortar , the 160 mm divisional mortar MT - 13 , which was similar to the NM in terms of weight , range and shell weight . Ammunition . The mortar used separate loading ammunition . The cartridge case could contain five different propellant charges . In addition to an old HE shell , two new HE - Frag shells were developed . The " draft 4139 " shell was produced in limited numbers and was no longer used by 1934 .