9mm is 0.355 and .38 is .0.357 diameter and I know the bullets are actually compatible with each other in reloading in their respective cases. (Though not recommended unless you’re an expert reloader)
Since both are the most popular cartridge for both semi-auto pistols and revolvers respectively, I’m curious if it’s a coincidence both happen to have the same diameter bullets. Was one based of the other? Is 0.355 just the best ballistically for a handgun?
A .38 S&W Special round (and the .38 Long Colt from which it is derived) is a heeled bullet (bullet is seated with a flange greater than the internal diameter of the cartridge) designed to be fired from a straight-walled cartridge in a revolver with 0.38 in bore chambers and headspaced from the rim, hence the diameter was dictated by the existing cartridge dimensions. The 9x19mm Parabellum is a taper-walled ‘rimless’ cartridge that was a blank sheet design for the 9mm version of the Luger P-08 to improve “stopping power” over the 7.65x21mm Parabellum version. The bullet diameter is 0.355 inches because that is the closest 0.001 inch increment to 9.00 millimeters.
The difference in diameters is functionally irrelevant in terms of ballistics, and indeed the ballistics of the standard pressure load of .38 Spl and 9mmP rounds of the same bullet weights are practically identical although the .38 Spl tended to be loaded with 158 gr bullets while the normal 9mmP hardball round was 124 gr.
Stranger
There are many cartridges in the 9mm/.38 family. It seems to hit a sweet spot considering power, recoil, number of rounds that can be carried, etc. In rifle cartridges the .30 caliber, or 7.62 is another grouping of cartridges with dozens of variations. The “Cartridges of the World” book is 1 1/2" thick with a different cartridge on every page.