The AK-47 is a Soviet-designed 7.62x39mm assault rifle developed between 1944–1946 by Mikhail Kalashnikov at Izhevsk Mechanical Works and has a reputation for its ruggedness, ease of manufacture, and reliability.
Made from mostly stampings, rivets and a few seal milled components, with a trigger sprung by wire and very loose part tolerances, the AK has several important traits:
1. The ability to digest even the worst ammunition
2. Extremely low cost and ease of manufacturing; you can make an AK from a shovel if you're stubborn enough
3. Very, VERY poor performance in mud. The loose tolerances of the AK allow shoddy, rough parts to still work, but also mean that the gun is (literally) full of holes, allowing the ingress of mud and dirt.