I got a loose leaf binder and started a gun book. In one section I list the guns as I buy them with a column for if/when I sell them. In this is the where, when and how much. One section just has serial numbers. In one section each gun has it's individual page with upgrades and how much was spent, where the parts came from, who did the mods, etc. You can have a section where you keep track of other gear such as holsters, mags, belts, range bag, etc. Again, how much you paid, where you bought, etc. I wish I would have started this from the get-go. I have some information that goes back a ways and other tabs that I just started. You could also do an ammo log per gun in this arrangement. Making comments on what you have learned, what tips you've picked up could also be helpful.
