It might seem like a small difference if you're not used to handling pistol cartridges, but if you are, that's like saying it's difficult to tell PHP from Perl.
Edit: Also, there is usually an imprint on the bottom of the cartridge that says "9MM LUGER".
I handle a lot of guns and that is a great analogy. We know that all we have to do is look for a ".php". But how many average people notice that type of stuff?
You're right that there are imprints on all rounds but seeing and remembering that is like getting a database error and remembering if it was Microsoft or MYSQL...You have to have a lot of experience and a keen memory.
You're assuming that the author is an average person with respect to gun knowledge. I don't know how much he knows about them, but the story seems to suggest he knows a little, right? Maybe relevant: he was a war correspondent in Iraq[1]. Or maybe he just likes guns?
Edit: Also, there is usually an imprint on the bottom of the cartridge that says "9MM LUGER".