and poaching use a screw thread to attach the chambered barrel to a receiver with some sort of breech and firing pin. The user unscrews the barrel from the receiver to expose the chamber to load a cartridge. The RN50 .50BMG single shot rifle uses a similar screwthread breech cap to allow an otherwise simple break action to contain a .50BMG round. Modern single-shots Although non-cartridge single-shot firearms are still made in hobbyist contexts (for example, replicas of antique guns), this discussion focuses on newer designs employing cartridges. Pistols The modern era of single-shot firearms is most visible in the realm of pistols. Remington introduced the single-shot bolt-action XP-100 pistol in 1963, which heralded the era of high-performance, high-velocity pistols. The .221 Fireball cartridge lived up to its name by reaching velocities of 2700 ft/s (823 m/s) from a 10.5" (26.7 cm) barrel. Essentially a shortened .222 Remington, the compact .221 Fireball delivered accuracy exceeding many rifles, out to ranges unheard of for other handguns. Even bigger than the XP-100, the 1967 introduction of the Thompson Center Arms Contender pistol changed handgun sports forever. The Contender was a break-open design that allowed barrels to be changed by the shooter in minutes. Available in calibers from .22 Long Rifle up to .45-70, and in barrel lengths of 8, 10, and 14 inches (20, 25, and 35.5 cm), the Contender could, in the right hands, handle any type of game, and delivered rifle-like accuracy to match the XP-100. Many other manufacturers make single-shot pistols, most based on the bolt-action rifle, with barrels generally ranging from 10 to 15 inches (25 to 38 cm). Single-shots dominate handgun metallic silhouette shooting, and single-shots are the most common handguns used for hunting. Single-shot pistols have sometimes found popularity among insurgents, resistance fighters, and street gangs. The mass-produced, low-cost Liberator pistol of World War II, which was manufactured and distributed by U.S. forces to Allied Resistance forces and Guerrilla fighters as an assassination pistol, is the most common example of a mass-produced single-shot pistol. More than a million units were produced and distributed freely and many remain in private hands. A few varieties of zip guns could also be considered single-shot pistols. In recent years these improvised firearms have become more common in the hands of criminals and insurgents, especially when manufactured firearms are difficult to acquire. Rifles Ruger In 1966, Sturm, Ruger introduced their first