The AKM has a barrel using a chrome-lined bore and 4 proper-hand grooves in a 240 mm (1 in 9.45 in) or 31.5 calibers rifling twist amount. The forward barrel trunnion has a non-threaded socket with the barrel and a transverse gap to get a pin that secures the barrel in place. The AKM's barrel is put in in the forward trunnion and pinned (rather th