You can use a 38 special round in a 357 pistol. From what I have read the 38 special 357 Magnum 9mm 380 all are about 36 caliber and came about from the 36 caliber cap and ball Navy Colt. Does any one know of a true 38 caliber? Below is what I have found in my research. 38 Special From Wikipedia, the free encyclopedia The .38 Smith & Wesson Special (commonly .38 Special, .38 Spl, , or .38 Spc, pronounced "Thirty-eight Special") is a rimmed, centerfire cartridge designed by Smith & Wesson. It is most commonly used in revolvers, although some semi-automatic pistols and carbines also use this round. The .38 Special was the standard service cartridge of most police departments in the United States from the 1920s to the early 1990s. In other parts of the world, particularly Europe, it is known by its metric designation 9x29mmR History Despite its name, its 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 .374 inch diameter, requiring "heel-based" bullets, the exposed portion of which was the same diameter as the cartridge case (see the section on the .38 Long Colt). The .380 ACP (Automatic Colt Pistol) (also referred to as the "9mm Short", "9mm Browning", "9mm Kurz", "9mm Corto" or "9x17mm") pistol cartridge is a rimless, straight-walled pistol cartridge developed by firearms designer John Browning. It was introduced in 1908 by Colt, and has been a popular self-defense cartridge ever since. The .36 caliber Navy revolver was much lighter than the contemporary Third Model Dragoon revolvers developed from the .44 Walker Colt revolvers of 1847 that had been designed to be carried in holsters on either side of a saddle pommel. It is an enlarged version of the .31 caliber pocket revolvers that evolved from the earlier Baby Dragoon, and like them is a mechanically improved and simplified descendant of the 1836 Paterson revolver. As the factory designation implied, the Navy revolver was suitably sized for carrying in a belt holster. It became very popular in North America at the time of western expansion. Colt's aggressive promotions distributed the Navy and his other revolvers across Europe, Asia and Africa. The .36 caliber (.375-.380-inch) round lead ball weighs 86 grains and, at a velocity of 1,000 feet per second, is comparable to the modern .380 pistol cartridge in power. Loads consist of loose powder and ball or bullet, metallic foil cartridges (early) and combustible paper cartridges (Civil War era), all combinations being ignited by a fulminate percussion cap applied to the nipples at the rear of the chamber. The .357 S&W Magnum, or simply .357 Magnum, is a revolver cartridge created by Elmer Keith, Phillip B. Sharpe, Colonel D. B. Wesson of firearms manufacturer Smith & Wesson, and Winchester. It is based upon Smith & Wesson's earlier .38 Special cartridge. The .357 Magnum cartridge was introduced in 1934, and its use has since become widespread.  Design The .357 Magnum was collaboratively developed over a period in the early to mid-1930s by a group of individuals in a direct response to Colt's .38 Super Automatic. At the time, the .38 Super was the only pistol cartridge capable of defeating automobile cover and the early ballistic vests that were just beginning to emerge in the post-World War I "Gangster Era." Tests at the time revealed that those vests defeated any handgun cartridge traveling at less than about 1000 ft/s. Colt's .38 Super Automatic just edged over that velocity and was able to penetrate car doors and vests that bootleggers and gangsters were employing as cover. Much credit for the .357's early development is given to hunter and experimenter Elmer Keith. Keith's early work in loading the .38 Special to increasingly higher pressure levels was made possible by the availability of heavy, target shooting-oriented revolvers like the Smith & Wesson 38/44 "Heavy Duty" and "Outdoorsman", .38-caliber revolvers built on .44-caliber frames. The .38-44 HV load, used the .38 Special cartridge loaded to a much higher velocity than standard .38 Special ammunition. The .38-44 revolvers were made by using a .44 Special size gun with the barrel and cylinder bored to .357 caliber (the true bullet diameter of the .38 Special).