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 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 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. 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" ("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