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The forcing cone is the constriction at the end of the chamber that forces the load down from chamber size to the size of your shotgun bore. If you hold the barrel up, point the muzzle toward the light and look through it from the chamber end, the forcing cone will appear as a short, dark ring, dark because the angle of the short forcing cone is severe and is hidden from the light. The distance from the beginning to the end is about 1/4". Most barrels come from the factory with this abrupt constriction, or forcing cone. When the shotgun is fired, the shot load is immediately FORCED, under tremendous pressure, to squeeze through the abrupt forcing cone to the smaller size of the bore. This results in some of the lead shot becoming deformed, making them ballistically unsound and causing them to fly out of the pattern. Lengthening the forcing cone by changing the abrupt angle to a more gentle, gradual constriction results in a new length of around 1 3/4". This optimum new length allows the shot to make a more gradual transition from chamber to bore size, reducing shot deformation and allowing more of the shot to remain in the pattern.
