In fact, many light machine guns (such as the Russian RPK) are adaptations of existing automatic rifle designs. A military's light machine guns are typically chambered for the same caliber ammunition as its service rifles. Generally, the difference between an automatic rifle and a machine gun comes down to weight, cooling system, and ammunition feed system. Rifles, with their relatively lighter components (which overheat quickly) and smaller capacity magazines, are incapable of sustained automatic fire in the way that machine guns are; they trade this capability in favor of increased mobility. Modern military rifles are fed by magazines, while machine guns are generally belt-fed. Many machine guns allow the operator to quickly exchange barrels in order to prevent overheating, whereas rifles generally do not. Most machine guns fire from an open bolt in order to reduce the danger of "cook-off", while almost all rifles fire from a closed bolt for accuracy. Machine guns are often crewed by more than one soldier; the rifle is an individual weapon. The term "rifle" is sometimes used to describe larger rifled crew-served weapons firing explosive shells, for example, recoilless rifles and naval rifles. In many works of fiction a rifle refers to any weapon that has a stock and is shouldered before firing, even if the weapon is not rifled or does not fire solid projectiles (e.g., a "laser rifle"). Historical overview The origins of rifling are as difficult to trace, but some of the earliest European experiments seem to have been carried out during the 15th century. Archers had long realized that a twist added to the tail feathers of their arrows gave them greater accuracy. Early muskets produced large quantities of smoke and soot, which had to be cleaned from the action and bore of the musket frequently, either through the action of repeated bore scrubbing, or a deliberate attempt to create "soot grooves" that would allow for more shots to be fired from the firearm. Some of the earliest examples of European grooved gun barrels were reportedly manufactured as early as 1440 and further developed by Gaspard Kollner of Vienna circa 1498, although other scholars allege they were a joint effort between Kollner and Augustus Kotter of Nuremberg circa 1520. Military commanders preferred smoothbore weapons for infantry use because rifles were much more prone to problems due to powder fouling the barrel and because they took longer to reload and fire