In October of 1941, a conscript into the Soviet army who rose to the rank of tank commander was wounded in the Battle of Brody. While he was recovering in the hospital, he heard a couple of other soldiers complaining about the poor quality of Soviet rifles, and thought back to his own difficulties with submachine guns. So he did what any engineering badass would have done.
He got off his ass and invented one of the most prolific, most reliable, most well-known rifles in the world: the AK-47.