I've never done this, but I've been acquiring a lot of magazines lately, and figured it can't hurt. While I've never had a problem magazine, failures are bound to happen eventually. How do you number them? Location? Paint pen or permanent marker? Obvious or as inconspicuous as possible? Bright color or subdued? I plan on using one color of paint pen, red, and mark them inconspicuously on the floor plate, based on the type of magazine. I don't think I have more than 9 of any type of magazine, so numbers 1-9 can be used for USGI aluminum as well as Black Magpul and FG Magpul will get their own set of numbers. 20 round mags will get the same treatment, as will pistol mags. I figure this will better allow me to track each mag, how many rounds I put through them, and get rid of them or repair them if they consistently fail.