Purpose
High pressures, such as those commonly encountered in maximum loadings of magnum revolver cartridges or rifle cartridges, often result in significant problems when coupled with cast or swaged lead bullets. It was long thought that the high temperatures melted the base of the bullet, but this is no longer thought to be the case. Instead, the high pressures allow propellant gas to escape past the bullet, causing gas cutting, which increases lead deposits in the barrel and unbalances the bullet. A gas check provides a thin layer of harder but still malleable metal on the base of the bullet that obturates to provide a seal and prevents the propellant gas leakage that causes gas cutting, and help the bullet grip the rifling.
While most cartridges operating at such high pressure use jacketed bullets, gas checked bullets are often less expensive, especially to a hand loader who can recycle lead to make cast bullets, and then must only pay for the gas check. Custom lead bullets, such as those for obsolete calibers, wildcat cartridges, or for special purposes, are easily made with inexpensive casting or swaging equipment. In contrast, manufacturing jacketed bullets requires far more expensive equipment to draw the jackets and swage in the core, so is generally limited to commercial ammunition producers. As a result, although it is possible for hobbyists to manufacture jacketed bullets, many of them take the easier option and use gas checked bullets instead.
Read more about this topic: Gas Check
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