In the field of firearms and airguns, obturation denotes necessary barrel blockage or fit by a deformed soft projectile (obturation 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), 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. Obturation in firearms ammunition With reference to firearms and air guns, obturation is the result of a bullet or pellet expanding or upsetting to fit the bore, or, in the case of a firearm, of a brass case expanding to seal against the chamber at the moment of firing. In the first case, this both seals the bullet in the bore, and causes the bullet to engage the barrel's rifling. In the second case, it seals the case in the chamber and prevents backward travel of gases against the bolt. The thin brass case easily seals the chamber, even in low pressure rounds like the .22 CB, but expanding or upsetting the bullet sufficiently for effective obturation requires sufficient pressure to deform the bullet material. The formula used to calculate the pressure required for solid base bullets is: Bullet's Brinell hardness