Hopefully it makes more sense to you now. You cant really assume much on the actual bullet diameter based on caliber. I only really started grasping at all the variations when I started playing with reloading. 38 special, 38 caliber bullet, same size as a 357mag. 45auto, .451, while 45 long colt, .452 in modern guns, .454 when dealing with prewar(preWWII) guns. Well at least thats what my old Lyman manual said.