Chamber and Forcing Cone of a Shotgun Barrel Drawing from: Gough Thomas, Shotguns & Cartridges, A & C Black, London, 1975.
Breech End Loaded Chamber Forcing Cone Bore, proper --->
Chamber - An area at the breech end of a barrel, of about the diameter of the cartridge for which the gun was intended, and into which the cartridge is inserted. The nominal length of a shotgun chamber will accommodate the loaded cartridge for which it was intended and allow for its crimp to open fully when the cartridge is fired. Although one can easily insert a longer-than-nominal-length loaded cartridge in a shotgun chamber, it is not advisable to do so because when it is fired the crimp will open into the forcing cone. Because of the taper of the forcing cone, the crimp will not be able to open fully and the gun will develop far greater pressure than it was designed to handle.
While most 12 gauge shotguns built today have nominal 2 3/4" chambers, this was not always the case. Prewar American guns and many modern English guns often have shorter chambers. It is important to know the length of a gun's chambers and to use the ammunition for which it was intended. To relieve the forcing cone by reaming, to allow use of longer and more powerful cartridges---removing metal at the same time as asking the gun to bear more pressure---is perilous.
Forcing Cone - In a shotgun barrel, A tapered area a few inches from the breech end, providing a transition between the chamber (approximately the diameter of the outside of a shotgun shell) to the bore proper (approximately the diameter of the inside of a shotgun shell). The forcing cone provides the transition between the exterior and the interiod diameters of the cartridge. Older shotguns usually have more abrupt forcing cones suitable for then-current thick-walled paper shells with fibre wads. Newer shotguns usually have more gradual, longer forcing cones suitable for thinner modern plastic shells with obturating plastic shot-cup wads.
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