youth rifle is a rifle designed or modified for fitting children or other small-framed shooters. A youth rifle is often a single-shot .22 caliber rifle, or a bolt-action rifle, although some youth rifles are semi-automatic. They are usually very light, with a greatly shortened length of pull, which is necessary to accommodate children. Youth stocks are available for many popular rifles, such as the Ruger 10/22, a semi-automatic .22 LR rifle, allowing a youth rifle to be made from a standard rifle by simply changing the stock. The typical ages of shooters for such rifles vary from about age 5+. Technical aspects Rifling The usual form of rifling was helical grooves in a round bore. Some early rifled firearms had barrels with a twisted polygonal bore. The Whitworth rifle was the first such type designed to spin the round for accuracy. Bullets for these guns were made to match the shape of the bore so the bullet would grip the rifle bore and take a spin that way. These were generally large caliber weapons, and the ammunition still did not fit tightly in the barrel. Many different shapes and degrees of spiraling were used in experimental designs. One widely produced example was the Metford rifling in the Pattern 1888 Lee–Metford service rifle. Although uncommon, polygonal rifling is still used in some weapons today, one example being the Glock line of pistols (which fire standard bullets). Many of the early designs were prone to dangerous backfiring, which could lead to the destruction of the weapon and serious injury to the person firing it. Barrel wear As the bullet enters the barrel, it inserts itself into the rifling, a process that gradually wears down the barrel, and also causes the barrel to heat up more rapidly. Therefore, some machine guns are equipped with quick-change barrels that can be swapped every few thousand rounds, or in earlier designs, were water-cooled. Unlike older carbon steel barrels, which were limited to around 1,000 shots before the extreme heat caused accuracy to fade, modern stainless steel barrels for target rifles are much more resistant to wear, allowing many thousands of rounds to be fired before accuracy drops. (Many shotguns and small arms have chrome-lined barrels to reduce wear and enhance corrosion resistance. This is rare on rifles designed for extreme accuracy, as the plating process is difficult and liable to reduce the effect of the rifling.)