The constant 1422 is an empirically determined factor used throughout the ammunition 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:
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.
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