When the Soviet Union was invaded by Nazi Germany in 1941 the Mosin–Nagant was the standard issue weapon of Soviet troops. As a result, millions of the rifles were produced and used in World War II by the largest mobilized army in history.
The Mosin–Nagant was adopted as a sniper rifle in 1932 and was issued to Soviet snipers. It served quite prominently in the brutal urban battles on the Eastern Front, such as the Battle of Stalingrad, which made heroes of snipers like Vasili Zaitsev and Ivan Sidorenko. The sniper rifles were very much respected for being very rugged, reliable, accurate, and easy to maintain. Finland also employed the Mosin–Nagant as a sniper rifle, with similar success. For example, Simo Häyhä is credited with killing 505 Soviet soldiers using his M28 Mosin–Nagant.
Female Soviet soldiers with sniper Mosin-Nagant rifles after the Battle of Königsberg.
In 1936, the 91/30 was again modified, this time to speed production. The receiver was changed from its octagonal shape (colloquially referred to as a "hex receiver" for reasons that are not clear) to an easier to make round receiver. When war with Germany broke out, the need to produce Mosin-Nagants in vast quantities led to a falling-off in finish of the rifles. The wartime Mosins are easily identified by the presence of tool marks and rough finishing that never would have passed the inspectors in peacetime. However, the basic functionality of the Mosins was unimpaired.
By the end of the war, approximately 17.4 million M91/30 rifles had been produced.