152 mm (5.9 inch)
The ML-20 fires 152 mm shells. It is primarily a carriage mounted weapon, relying on a truck or artillery tractor for movement, but many were used as the primary armament for the colossal ISU-152 self-propelled guns. It's main purpose was use as a infantry support gun and long range attack howitzer, but some were used anti-tank guns. The ML-20 proved to be very ineffective in this role due it's smooth-bore barrel. Another version of the ML-20 with a rifled barrel was developed for this role instead.
The ML-20 was first developed in 1937 for the Red Army. It was used extensively, among other weapons, throughout the early stages of Russia's war with Germany, from 1941 to 1943. In 1943, a newer model of the ML-20 was developed, and was named the 1937/43. This weapon was even more effective than it's predecessor, and remained in service with Soviet forces until the end of the war and beyond.