Obturate
Encyclopedia
Obturate means to block or obstruct. With reference to firearms and airguns, obturation refers to the process of a bullet
or pellet
, 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
. 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. 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.
s and air gun
s, 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 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:
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.
Bullet
A bullet is a projectile propelled by a firearm, sling, or air gun. Bullets do not normally contain explosives, but damage the intended target by impact and penetration...
or pellet
Pellet (air gun)
An air gun pellet is, for the most part, a non-spherical projectile designed to be fired from an air gun. But this is not always the case.Pellets differ from bullets used in firearms because of the pressures encountered; firearms operate at pressures of thousands of atmospheres, while airguns...
, 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 process of making helical grooves in the barrel of a gun or firearm, which imparts a spin to a projectile around its long axis...
. 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. For shotgun shell
Shotgun shell
A shotgun shell is a self-contained cartridge loaded with lead shot or shotgun slug designed to be fired from a shotgun....
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 firearmFirearm
A firearm is a weapon that launches one, or many, projectile at high velocity through confined burning of a propellant. This subsonic burning process is technically known as deflagration, as opposed to supersonic combustion known as a detonation. In older firearms, the propellant was typically...
s and air gun
Air gun
An air gun is a rifle , pistol , or shotgun that fires projectiles by means of compressed air or other gas, in contrast to a firearm, which burns a propellant. Most air guns use metallic projectiles as ammunition. Air guns that only use plastic projectiles are classified as airsoft...
s, obturation is the result of a bullet
Bullet
A bullet is a projectile propelled by a firearm, sling, or air gun. Bullets do not normally contain explosives, but damage the intended target by impact and penetration...
or pellet
Pellet (air gun)
An air gun pellet is, for the most part, a non-spherical projectile designed to be fired from an air gun. But this is not always the case.Pellets differ from bullets used in firearms because of the pressures encountered; firearms operate at pressures of thousands of atmospheres, while airguns...
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 process of making helical grooves in the barrel of a gun or firearm, which imparts a spin to a projectile around its long axis...
. 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 9.80665 N/kgf×106 mm²/m² = [N/m²] = pressure in pascalsPascal (unit)The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...
- Bullet's BHN x 1422 = pressure in pounds per square inchPounds per square inchThe pound per square inch or, more accurately, pound-force per square inch is a unit of pressure or of stress based on avoirdupois units...
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:
| 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 . They are often used in handgun and airgun competitions...
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 chambers and at least one barrel for firing. The first revolver ever made was built by Elisha Collier in 1818. The percussion cap revolver was invented by Samuel Colt in 1836. This weapon became known as the Colt Paterson...
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. The raised areas of the rifling are called "lands," which make contact with the projectile , imparting spin around an axis corresponding to the...
and magnum pistol
Pistol
When distinguished as a subset of handguns, a pistol is a handgun with a chamber that is integral with the barrel, as opposed to a revolver, wherein the chamber is separate from the barrel as a revolving cylinder. Typically, pistols have an effective range of about 100 feet.-History:The pistol...
cartridges, lead bullets are often covered in copper or another harder alloy. These bullets are generally designed to swage
Swage
Swaging is a forging process in which the dimensions of an item are altered using a die or dies, into which the item is forced. Swaging is usually a cold working process; however, it is sometimes done as a hot working process....
to fit upon firing.