The AK-47 quickly became famous and spread all around the world because it was simple to fire, clean and maintain, and also because of its reliability, meaning that it can be fired for a long time without jamming. The AK-47 and its successors continue to be used by many of the world's armies. Many terrorist and insurgent groups also use the AK-47. It is a cheap, reliable, and easy-to-use weapon. The AK-47 was also available with a folding stock, the AKS-47, and a shortened version with the AKS74 folding stock, the AKMSU (used by armoured vehicle crews), although this was soon replaced by the AKS74U, which fires the 5.45 cartridge of the AK-74. There was also a light machine gun variant with a longer barrel and different shaped stock called the RPK.