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.408 Cheyenne Tactical
The .408 Cheyenne Tactical (or .408 CheyTac) is a specialized rimless, bottlenecked, centerfire cartridge for military long-range sniper rifles that was developed by Dr. John D. Taylor and machinist William O. Wordman. The round was designed with a possible military need for a cartridge for anti-personnel, anti-sniper and anti-materiel roles with a (supersonic) precision range of 2,200 yards (2,000 m). It is offered as a competitor to the most common military NATO long-range service cartridges such as .338 Lapua Magnum and the .50 BMG.
== History ==
The .408 Cheyenne Tactical is based on the .400 Taylor Magnum, which is based on a modified .505 Gibbs, necked down to 0.408 inches (10.36 mm). The .505 Gibbs is an old English big game cartridge that was designed to accommodate 39,160 psi (270 MPa) pressure. One of the disadvantages to these old cartridge cases intended for firing cordite charges instead of modern smokeless powder is the thickness of the sidewall just forward to the web. During ignition, the cartridge's base, forward to the bolt face, is not supported. The case is driven back against the bolt face which results in the stretching of the case, particularly the sidewall immediately forward of the web. When the sidewall resists the outward expansion against the chamber, the pressure stretches the case thereby increasing its length resulting in the sidewall becoming thinner at that stretch point.
In the .408 CheyTac cartridge casing design particular attention was directed toward thickening and metallurgically strengthening the case's web and sidewall immediately forward to the web to accommodate high chamber pressures. In modern solidhead cases, the hardness of the brass is the major factor that determines a case's pressure limit before undergoing plastic deformation. Lapua Ltd. solved this problem when they used the .416 Rigby as the parental case to the .338 Lapua Magnum. They created a hardness distribution ranging from the head and web (hard) to the mouth (soft) as well as a strengthened (thicker) case web and sidewall immediately forward of the web. This method results in a very pressure resistant case.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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