Looking at a cartridge from a few feet away, anyone who's a gun geek and has decent eyesight can probably tell you that a 9mm cartridge is either that, a .40 S&W, or something very obscure. Also though, almost all handgun cartridges have their caliber stamped on the rim. I just checked to see from how far away I can read the markings on my Federal .40 (I don't own any 9mm, but it should be basically comparable). In good light and at just the correct angle, I can read it from about arm's length. Otherwise I have to hold it close.