Obturation

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Obturation is the necessary barrel blockage or fit in a firearm or airgun created by a deformed soft projectile.^[1] A bullet or pellet made of soft material and often with a concave base will flare under the heat and pressure of firing, filling the bore and engaging the barrel's rifling. The mechanism by which an undersized soft-metal projectile enlarges to fill the barrel is, for hollow-base bullets, expansion from gas pressure within the base cavity and, for solid-base bullets, "upsetting"—the combined shortening and thickening that occurs when a malleable metal object is struck forcibly at one end.

Obturation is achieved in shotgun shells (which have multiple pellets much smaller than the barrel bore) by placing a plastic wad or biodegradable card of the same diameter as the barrel between the propellant powder and the pellets.

In a cartridge - such as a bullet or cased artillery shell - "obturation" is initiated by the action of a soft metallic cartridge case being pressed outwards against the chamber walls by the high pressure of the internal gases. This creates a self-sealing effect that prevents gasses from leaking out of the breech mechanism.^[2] The difficulty with leakage was one of the major obstacles to the early adoption of the breech-loading firearm, as it lowered pressures (and hence velocity), and also created danger or irritant to the shooter. Although there were early paper-cartridge breechloaders, the self-obturating nature of metallic cartridges (along with their waterproof nature) led to their rapid and almost universal adoption, in spite of their much greater cost, solving as they did the leakage problem without requiring a complex built-in sealing system.