In the field of firearms and airguns, obturation denotes necessary barrel blockage or fitment by a deformed soft projectile ( in general is closing up an opening). 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. 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. More importantly, "obturation" refers to 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 which prevents gasses from leaking out of the breech mechanism, without requiring a complex built-in sealing system. This 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), but it also created danger or irritant to the shooter. Although there were early paper-cartridge breechloaders, the self-obdurating nature of metallic cartidges (along with their waterproof nature) led to their rapid and almost universal adoption, in spite of their much greater cost.