Obturation - Obturation in Firearms Ammunition

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 BHN x 9.80665 N/kgf×106 mm²/m² = = pressure in pascals

Bullet's BHN x 1422 = pressure in pounds per square inch

The constant 1422 is a mathematically derived number. The number comes from converting the pressure in kgf/mm² (which are the units used to measure BHN) to lbf/in² (which are the units used to measure cartridge pressure). That is:

Conversion factor = 25.4 (mm/in) x 25.4 (mm/in) x 2.2046 (lbf/kgf) = 1,422.

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 slugs 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.