Obturation

Article summary

Obturation is the necessary barrel blockage or fit in a firearm or airgun created by a deformed soft projectile. 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. 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.…

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