Obturation

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:

Bullet's Brinell hardness number (BHN) x 9.80665 N/kgf×10⁶ mm²/m² = [N/m²] = pressure in pascals

Bullet's BHN x 1422 = pressure in pounds per square inch[3]

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:

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 or flexible polymers[4] 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.[5] 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 be compressed upon firing as they leave the chamber and enter barrel. This seals the bullet to the interior of the barrel and engages the rifling.

The same principles apply to artillery ammunition; guns are traditionally categorized (in English-speaking countries) as "BL" or "QF" (for "Breechloading" or "Quick Firing"). "BL" guns are the older style, and typically use propellant stored in a number of fabric bags, the number of which can be adjusted to vary the range in some cases, as well as a separate shell that is rammed before the powder charge is placed into the breech. Large-caliber naval guns are traditionally of the "BL" type. Lacking a cartridge case to seal the breech, these guns require a complex and strong breech mechanism to safely seal the breech during firing. "QF" guns were a later development, usually of small to medium caliber, and came about from the need for rapid firing guns to counter torpedo boats and other small, nimble threats. A QF gun fires metallic-cased ammunition, generally with the shell fixed to it, as a single unit, just like a firearm cartridge. This allows a simpler breech mechanism, such as a sliding block breech, which when coupled with the easier ammunition handling and ramming procedures, allows a much higher rate of fire. Larger guns are most often BL guns, because the ammunition is so large and heavy, that even adding a metallic cartridge case would add a great deal of weight and make handling more difficult, even if the shell and propellant were loaded separately. There have been exceptions, however; some German naval guns during WWII used fabric powder bags with a thin brass base section, to provide obturation to the breech without adding too much weight, similar to a modern plastic shotshell with a "low-brass" base for sealing and extraction.

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