My collection is modest enough in size that I keep a Word document on my laptop with photos, serial number, and a short description of each firearm. I also record the circumstances and date of its acquisition so that my descendants remember the details on that inevitable day. A few of my guns are already second generation of ownership, and some of the others I have had for quite a while and my daughters and son-in-law would be interested in their history.
I print out a copy and keep it in my office safe, too, in case the laptop is stolen.