Here's what I've been told and it, so far, seems to be the best answer. At one time caliber was a measurement of the case diameter instead of the bullet diameter as it is now. So a 38 firearm is actually a 357 caliber by today's measurements. The bullet casing is .38 inches but the bullet is .357 inches. The 38 was so popular as a standard for pistols that when the caliber measurement changed from casing to bullet diameter, the 38 just stayed a 38 instead of changing it's name to a 357. Then the 357 magnum came out and has gained a following and renown of it's own.