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Hub AI
Obturation AI simulator
(@Obturation_simulator)
Hub AI
Obturation AI simulator
(@Obturation_simulator)
Obturation
Obturation is the necessary barrel blockage or fit in a firearm or airgun created 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.
Obturation is achieved in shotgun shells (which have multiple pellets much smaller than the barrel bore) by placing a plastic wad or biodegradable card of the same diameter as the barrel between the propellant powder and the pellets.
In a cartridge—such as a bullet or cased artillery shell—"obturation" is initiated by the action of a soft metallic cartridge case being pressed outwards against the chamber walls by the high pressure of the internal gases. This creates a self-sealing effect that prevents gasses from leaking out of the breech mechanism. The difficulty with leakage was one of the major obstacles to the early adoption of the breech-loading firearm, as it lowered pressures (and hence velocity), and also created danger or irritant to the shooter. Although there were early paper-cartridge breechloaders, the self-obturating nature of metallic cartridges (along with their waterproof nature) led to their rapid and almost universal adoption, in spite of their much greater cost, solving as they did the leakage problem without requiring a complex built-in sealing system.
Artillery shells rely on a driving band—a band of soft metal built into the shell casing near its base—to provide a seal between the barrel and projectile. Where the propellant charge is not enclosed in a cartridge, the breech itself must be fitted with an obturating mechanism, usually the de Bange obturator whereby a metal "nose cone" at the front of the breech screw compresses what was originally a grease-impregnated asbestos ring. This ring consequently expands sideways against the barrel wall and provides the required gas seal.
Obturation in firearms and air guns 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 or flexible polymers in conjunction with a thin, concave base designed to expand more easily. Some airgun projectiles are composed of materials harder than the soft alloys traditionally relied upon; these instead commonly make use of elastomer rings capable of providing sufficient obturation under pressure. 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.
Obturation
Obturation is the necessary barrel blockage or fit in a firearm or airgun created 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.
Obturation is achieved in shotgun shells (which have multiple pellets much smaller than the barrel bore) by placing a plastic wad or biodegradable card of the same diameter as the barrel between the propellant powder and the pellets.
In a cartridge—such as a bullet or cased artillery shell—"obturation" is initiated by the action of a soft metallic cartridge case being pressed outwards against the chamber walls by the high pressure of the internal gases. This creates a self-sealing effect that prevents gasses from leaking out of the breech mechanism. The difficulty with leakage was one of the major obstacles to the early adoption of the breech-loading firearm, as it lowered pressures (and hence velocity), and also created danger or irritant to the shooter. Although there were early paper-cartridge breechloaders, the self-obturating nature of metallic cartridges (along with their waterproof nature) led to their rapid and almost universal adoption, in spite of their much greater cost, solving as they did the leakage problem without requiring a complex built-in sealing system.
Artillery shells rely on a driving band—a band of soft metal built into the shell casing near its base—to provide a seal between the barrel and projectile. Where the propellant charge is not enclosed in a cartridge, the breech itself must be fitted with an obturating mechanism, usually the de Bange obturator whereby a metal "nose cone" at the front of the breech screw compresses what was originally a grease-impregnated asbestos ring. This ring consequently expands sideways against the barrel wall and provides the required gas seal.
Obturation in firearms and air guns 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 or flexible polymers in conjunction with a thin, concave base designed to expand more easily. Some airgun projectiles are composed of materials harder than the soft alloys traditionally relied upon; these instead commonly make use of elastomer rings capable of providing sufficient obturation under pressure. 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.