Gas check

[1] Gas checks are most commonly found in the form of a thin cup or disc made of a ductile metal.

The shallow cup-shaped check is mechanically attached to the reduced shank of the bullet by swaging.

Custom lead bullets, such as those for obsolete calibers, wildcat cartridges, or for special purposes, are easily made with inexpensive casting or swaging equipment.

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.

Gas checks allow these bullets to be pushed to higher velocities without undue fouling of the barrel and attendant problems.