To clarify the issue of the actual diameter of handgun bullets, a 9mm Parabellum barrel inner bore diameter, measured in the grooves, not in the slightly raised spin-imparting lands, is indeed 9mm, within standard manufacturing tolerances. This is equivalent to about .355 inches.
.357 magnums and so-called .38 Specials have bullet diameters of about .357 inches, almost identical to the 9mm. .357 Magnums have more powder in a larger case. The .38 Special is misnamed because some older black powder pistols did indeed fire a .38 lead ball, and the designation continued with equivalent smokless powder weapons, though if you look at old Civil War firearms, some are correctly designated as .36 caliber.
Modern cartridges vary widely, depending on bullet weight and the amount and exact type of powder, its burn rate, etc. Wartime loads made by the Germans for their 9mm submachine guns are so hot they can turn a pistol to junk with one shot. Likewise with some WW2 era .45 ACP rounds designed for guns like the Thompson, which carry a big warning "For Submachinegun Use ONLY".
Generally 9mm handgun rounds are more powerful than .38 Specials, but not as powerful as the .357 Magnum. There are all kinds of extra hot modern pistol loads, designated with +, +++, and other warnings that you may have a bad experience by trying to fire them from an older pistol. The biggest difference between the 9mm, the.38 Special (really an anemic .357), and the .357 Magnum is the fact that the 9mm, like the .45 ACP (Automatic Colt Pistol), has a rimless case, designed to be fired in a semi-auto pistol, not from a revolver. Owners of .357 Magnums know that they can fire the milder .38 Specials from their revolver, though the reverse is not possible.
