Obturate

Obturate

[ob-tuh-reyt, -tyuh-]
Obturate means to block or obstruct. In conventional English usage, obturation refers specifically to the condition of being obstructed or occluded or the action of blocking, stopping, or filling something up. 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 (e.g. a rivet) is struck forcibly at one end.

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 1422 = Pounds per square inch

The constant 1422 is an empirically determined factor used throughout the ammunition and reloading industry; it is specifically tailored for calculating the pressure required for expansions that occur in internal ballistics and should not be used for other purposes. Below is a chart containing various bullet alloys, the BHN, and the PSI required to expand a bullet to the bore:

Material BHN Pressure
(psi) (MPa)
Pure lead 5 7,110 49
1:20 tin/lead 10 14,200 98
1:10 tin/lead 11.5 16,400 113
Pure copper 40 56,900 392

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.

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