.40 caliber has diameter of .40 and 9mm has diameter of .355. The .40 uses bullets as low as 135 grain and as high as 200 grain. The 9mm generally likes bullets between 115 and 147 grain best, although there are exceptions. The .40 is thought to be a more effective "stopper" in loads traditionally available to the public, whereas the 9mm does not really get into the exceptional stopping power percentages unless you have the +P+ law enforcement only loads. Some might argue with this, but everyone has an opinion on this topic. If you are comfortable with a 38 special revolver, the 9mm is for you. If you like .45s and .357 Magnums, you will probably prefer the .40 S&W.