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The Winchester Model 21 is a deluxe side by side shotgun. The shotgun's initial production run from 1931 through 1959 yielded approximately 30,000 guns. Winchester Repeating Arms Company ceased the main production line of this shotgun in 1960 and the Model 21 was sourced to the Winchester Custom Shop until the gun's retirement in 1991. New Winchester Model 21 production continues under license to Connecticut Shotgun Manufacturing Company.

The Winchester Model 21 action is of a typical breech loading shotgun, commonly called a break or hinge action. Like all quality double guns, production is time-consuming due to the laborious process of joining the barrels to produce an identical convergence of shot. The Model 21 was Winchester's effort to make a quality side by side shotgun to rival those of high-end makers such as Parker and Fox; financial troubles plagued the gun's development until the Western Cartridge Company purchased Winchester Repeating Arms in 1931. The Model 21 has a considerable collectors following as it is regarded as almost a custom-made shotgun.