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Gases from the burning powder (deflagration) expand the case to seal it against the chamber wall.

The projectile is then pushed in the direction that has least resistance to this pressure, down the barrel.

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The length of a cartridge case may also be designated in millimeters.

The stated caliber of any given ammunition or firearm may not necessarily correlate to the exact dimensions.

Steel cases found in ammunition are often lacquered, or coated in a thin layer of polymer or copper (referred to as copper-washed) to protect the steel from corrosion prior to firing, after which rusting starts from the inside.

Some ammunition is also made with aluminum cases (see picture).

The .38 Special actually has a bullet diameter of 0.357 inches (9.1 mm) (jacketed) or 0.358 inches (9.1 mm) (lead) while the case has a nominal diameter of 0.380 inches (9.7 mm) to allow the projectile to fit within a case with a wall thickness of half of the 23/1000 of an inch difference.

The .357 Magnum is a direct evolution of the .38 Special, but differently named, and no reference is made to the longer case except by the name 'Magnum'.Most high-powered firearms have relatively small projectiles moving at high speeds.This is because while bullet energy increases in direct proportion to bullet weight, it increases more so in proportion to the square of bullet velocity.After the projectile leaves the barrel the pressure drops, allowing the cartridge case to contract slightly easing its removal from the chamber.Automatic and semiautomatic firearms, which extract and eject the case automatically as a part of their operating cycle, sometimes damage the case in the process of ejection.Where two numbers are together, the second number can contain a variety of meanings.

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