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Obturate means to block or obstruct. With reference to firearms and airguns, obturation refers to the process of a bullet
Bullet

A bullet is a hard projectile propelled by a firearm, Sling , or air gun and is normally made from metal. A bullet does not contain explosives, but damages the intended target by tissue or mechanical disruption through impact or penetration....
 or pellet
Pellet

Pellet are small particles typically created by compressing an original material. Specific items often termed 'pellet' include:* Pelletizing is the industrial process used to create pellets, using a pellet mill...
, made of soft material and often with a concave base, flaring under the pressure of firing to seal the bore and engage the barrel's rifling
Rifling

Rifling is the helix-shaped pattern in the Gun barrel of a gun or firearm, which imparts a spin to a projectile around its long axis. This spin serves to gyroscope stabilize the projectile, improving its Aerodynamics stability and accuracy....
. 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 is struck forcibly at one end.

reference to firearm
Firearm

A firearm is a tool that projects either single or multiple projectiles at high velocity through a controlled explosion. The firing is achieved by the gases produced through rapid, confined combustion of a propellant....
s and air gun
Air gun

An air gun is a rifle, pistol, or shotgun which fires projectiles by means of compressed pneumatic or other gas, in contrast to a firearms which burn a propellant....
s, obturation is the result of a bullet
Bullet

A bullet is a hard projectile propelled by a firearm, Sling , or air gun and is normally made from metal. A bullet does not contain explosives, but damages the intended target by tissue or mechanical disruption through impact or penetration....
 or pellet
Pellet

Pellet are small particles typically created by compressing an original material. Specific items often termed 'pellet' include:* Pelletizing is the industrial process used to create pellets, using a pellet mill...
 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.






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Obturate means to block or obstruct. With reference to firearms and airguns, obturation refers to the process of a bullet
Bullet

A bullet is a hard projectile propelled by a firearm, Sling , or air gun and is normally made from metal. A bullet does not contain explosives, but damages the intended target by tissue or mechanical disruption through impact or penetration....
 or pellet
Pellet

Pellet are small particles typically created by compressing an original material. Specific items often termed 'pellet' include:* Pelletizing is the industrial process used to create pellets, using a pellet mill...
, made of soft material and often with a concave base, flaring under the pressure of firing to seal the bore and engage the barrel's rifling
Rifling

Rifling is the helix-shaped pattern in the Gun barrel of a gun or firearm, which imparts a spin to a projectile around its long axis. This spin serves to gyroscope stabilize the projectile, improving its Aerodynamics stability and accuracy....
. 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 is struck forcibly at one end.

Obturation in firearms ammunition

With reference to firearm
Firearm

A firearm is a tool that projects either single or multiple projectiles at high velocity through a controlled explosion. The firing is achieved by the gases produced through rapid, confined combustion of a propellant....
s and air gun
Air gun

An air gun is a rifle, pistol, or shotgun which fires projectiles by means of compressed pneumatic or other gas, in contrast to a firearms which burn a propellant....
s, obturation is the result of a bullet
Bullet

A bullet is a hard projectile propelled by a firearm, Sling , or air gun and is normally made from metal. A bullet does not contain explosives, but damages the intended target by tissue or mechanical disruption through impact or penetration....
 or pellet
Pellet

Pellet are small particles typically created by compressing an original material. Specific items often termed 'pellet' include:* Pelletizing is the industrial process used to create pellets, using a pellet mill...
 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
Rifling

Rifling is the helix-shaped pattern in the Gun barrel of a gun or firearm, which imparts a spin to a projectile around its long axis. This spin serves to gyroscope stabilize the projectile, improving its Aerodynamics stability and accuracy....
. 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
.22 CB

.22 CB Cap is a variety of .22 caliber rimfire ammunition which has a very small propellant charge , resulting in a low muzzle velocity of between 350 and 700 ft/s ....
, 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
Ammunition

Ammunition, often referred to as ammo, is a generic term derived from the French language la munition which embraced all material used for war , but which in time came to refer specifically to gunpowder and artillery....
 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
Wadcutter

A wadcutter is a special-purpose bullet specially designed for shooting paper targets, usually at close range and at subsonic velocities typically under 800 ft/s ....
 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
Internal ballistics

Internal ballistics, a subfield of ballistics, is the study of a projectile's behavior from the time its propellant's igniter is initiated until it exits the gun barrel....
). For example, it is not uncommon for revolver
Revolver

A revolver is a repeating firearm that has a Cylinder containing multiple Chamber and at least one Gun barrel for firing. As the user cocks the hammer , the cylinder revolves to align the next chamber and round with the hammer and barrel, which gives this type of firearm its name....
 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
Rifle

A rifle is a firearm designed to be fired from the shoulder, with a barrel that has a helical groove or pattern of grooves cut into the barrel walls....
 and magnum pistol cartridges, lead bullets are often covered in copper or another harder alloy. These bullets are generally designed to swage
Swage

Swaging is a metal-forming technique in which the dimensions of an item are altered using a die or dies, into which the item is forced. Swaging is a forging process, usually performed cold work; however, it can be done hot....
 to fit upon firing.

See also

  • Driving band
    Driving band

    The driving band or rotating band is part of an artillery Shell_ , a band of soft metal near the middle of the shell, typically made of gilding metal, copper or lead....
  • Gas check
    Gas check

    A gas check is a device used in some types of firearms ammunition. Gas checks are used when non-full metal jacket bullet are used in internal ballistics cartridge , to prevent the buildup of lead in the gun barrel and aid in accuracy....