Obturation

In the field of firearms and airguns, obturation refers to barrel blockage caused by a deformed soft projectile. A bullet or pellet, made of soft material and often with a concave base, may flare under the pressure of firing sealing 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, due to 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. For shotgun shells which have multiple pellets much smaller than the barrel bore, obturation is achieved by placing a plastic wad or biodegradable card of the same diameter as the barrel between the propellant powder and the pellets.