High Quality Content by WIKIPEDIA articles! Obturate means to block or obstruct. With reference to firearms and airguns, obturation refers to the process of a bullet or pellet, made of soft material and often with a concave base, flaring under the pressure of firing to seal the bore and engage 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.