Obturation In the field of firearms and airguns , obturation denotes necessary barrel blockage or fitment 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 . For shotgun shell s 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 . Obturation in firearms ammunition . With reference to firearm s 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 : The conversion factor of 1422 is mathematically derived in order to convert the pressure in kgf / mm² ( the units used to measure BHN ) to lbf / in² ( the units used to measure cartridge pressure ) . That is : Note that this number should only be used with cast lead plain - base bullets . It does not apply to jacketed or gas - check cast bullets . Below is a chart containing various bullet alloys , the BHN , and the PSI required to expand a bullet to the bore : Pure lead is very soft , and can be expanded or upset by most firearm cartridges , but the pressures required are higher than those encountered in most airguns . To allow obturation in airguns , pellets use soft alloys in conjunction with a thin , concave base designed to expand more easily . Some firearms ammunition , such as Foster slug s and hollow base wadcutter bullets , also use a hollow base to allow the bullet to expand and conform to a barrel ' s irregularities , even as the chamber pressure drops as the bullet travels down the barrel ( see internal ballistics ) . For example , it is not uncommon for revolver barrels to have a slight constriction at the breech end where they thread into the revolver ' s frame ; a hollow base bullet will expand to fill the larger diameter of the barrel after passing through the constriction . To prevent excessive deformation in high pressure rifle and magnum pistol cartridges , lead bullets are often covered in copper or another harder alloy . These bullets are generally designed to swage to fit upon firing .