Part of the difference may be that most polygonal rifling is produced by hammer forging the barrel around a mandrel containing a reverse impression of the rifling. Hammer forging machines are tremendously expensive, far out of the reach of custom gunsmiths (unless they buy pre-rifled blanks), and so are generally only used for production barrels by large companies. The main advantage of a hammer forging process is that it can rifle, chamber, and contour a bored barrel blank in one step. First applied to rifling in Germany in 1939, hammer forging has remained popular in Europe, but was only later used by gunmakers in the United States. The hammer forging process produces large amounts of stress in the barrel that must be relieved by careful heat treatment, a process that is less necessary in a traditionally cut or button rifled barrel. Due to the potential for residual stress causing accuracy problems, precision shooters in the United States tend to avoid hammer forged barrels, and this limits them in the type of available rifling. From a practical standpoint, any accuracy issues resulting from the residual stresses of hammer forging are extremely unlikely to be an issue in a defense or service pistol, or a typical hunting rifle.