A very popular revolver caliber… the “thirty-eight special” was Standard issue for the FBI for a very long time, these rounds have manageable recoil with a slower, heavier bullet than say a 9mm pistol. The .357 Magnum round is identical except being slightly longer when compared to a .38 special. You can safely fire a .38 special in a .357 Magnum but don’t try the other way around.