3x5Cards
I've been keeping 3x5 cards for years (35 to be exact) once I got more than one gun. I list make, model, caliber, significant demensions, s/n, where I bought it and how much. When I sell it I list to whom and how much.
Of course in a separate file, in a separate room of the house, I have all the receipts.
I also started something for my wife called a "love letter". In it I details all the things she needs to know about if I die. This includes a listing of all guns and their value. I update it every New Years. HINT: This is good thing for all spouses to do. List all financial accounts, and assets (like guns and ham radios) that she might not be acquainted.
I may look into this gunsafe program.