38 pistol & revolver

Despite its name, the .38 Special caliber is actually .357–.358 inches (9.0678 mm), with the ".38" referring to the approximate diameter of the loaded brass case. This came about because the original .38-caliber cartridge, the .38 Short Colt, was designed for use in converted .36-caliber cap-and-ball (muzzleloading) Navy revolvers, which had cylindrical firing chambers of approximately 0.374-inch (9.5 mm) diameter, requiring heeled bullets, the exposed portion of which was the same diameter as the cartridge case (see the section on the .38 Long Colt).
Except for its length, the .38 Special case is identical to that of the .38 Long Colt, and to the .357 Magnum which was developed from the earlier cartridge in 1935. This allows the .38 Special round to be safely fired in revolvers chambered for the .357 Magnum. The reverse, however, is not true; the .357 Magnum case was specifically designed to be longer than that of the .38 Special so that .357-caliber ammunition would not chamber in .38-caliber weapons, which are not designed for the greatly increased pressure of the magnum rounds.
The .38 Special was introduced in 1899 as an improvement over the .38 Long Colt which, as a military service cartridge, was found to have inadequate stopping power against the wooden shields of charging Moros during the Philippine-American War.Most hand-loading manuals and other references date the cartridge to 1902 and the Smith & Wesson Military and Police revolver variation of that year.