I use the KeePass password database because they have a version that runs on my BlackBerry and it allows me to keep them in synch. I used to use PasswordSafe by Bruce Schneier, a security expert, it also has a Java version, but I had trouble trying to run it on my BB. It probably would work fine in Linux.
I like your idea on you firearm inventory. Too bad my inventory is so small it really doesn't take too much to keep track of it.