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A gas check is a gasket type component of firearms ammunition. Gas checks are used when non-jacketed bullets are used in high pressure cartridges. The use of a gas check inhibits the buildup of lead in the gun barrel and improves accuracy.
While most commercial cartridges operating at high pressure use jacketed bullets, gas checked bullets are popular with handloading who can recycle lead to make , 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 projectile 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.
The other common use of gas checked bullets is in obsolete military rifles. Many of these rifles used calibers that were unique to the rifle; low levels of commercial production and dwindling supplies of surplus ammunition quickly result in high ammunition prices. Many of these rifles use unusual bore diameters; for example, both the British 7.7×56mmR and Soviet 7.62×54mmR rifle cartridges use bore diameters larger than US .30 caliber (7.62 mm) standard bullets, resulting in a much smaller supply of suitable bullets—often just a single full metal jacket bullet design in the weight used by the military loading. Custom made bullets also allow the bullet to be carefully sized to match the bore, which can vary considerably in surplus rifles, and provide both more accuracy and more flexibility. Gas checks allow these bullets to be pushed to higher velocities without undue fouling of the barrel and attendant problems.
An unusual example is the Benelli CB M2 submachine gun is unusual that it uses a ramrod sealed with gas rings instead of a bolt to chamber the 9x25mm AUPO caseless rounds. It uses an open-bolt type operation that operates in conjunction with a side mounted firing pin/hammer outside the chamber at the rear of the barrel to strike the primer of the rounds.
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