The AK-47 or Kalashnikov is a automatic rifle designed by the Soviet Union in the last year of WWII. It has since been manufactured in dozens of countries with multiple variants ranging from fine craftsmanship to sketchy workmanship.
Part of why the AK-47 is so popular is its reliability. The gun has been dragged through blazing deserts, cold tundra, and wet jungles and fires reliably all the way through. It is cheap and simple to make but lasts forever.