The "47" in the name of the famous AK-47 assault rifle comes from the year that the prototype weapon was completed.
The name AK-47 is derived from the Russian words “Avtomat Kalashnikova”, in honor of its automatic firing capabilities and its principal designer, Mikhail Kalashnikov. The 47 denotes the year 1947, when the trials started on the version of the rifle that was finally approved for adoption by the Soviet armed forces soon after.
Mikhail Kalashnikov used to be a tank commander during W\WII, but was injured and had to quit. Anyway, he continued his military career by becoming a weapons designer. As legend has it, he was in a hospital in 1942 when he heard two soldiers complaining about the quality of their rifles. Mikhail decided to change this, and 5 years later his first rifle was produced.