I use Evernote. I drop in all the relevant information regarding each gun into a separate note. I can then attach photos of the gun to the note, along with numerous other items (PDF of owners manual, links to manufacturers website, recall notices, etc). I have a note for each gun in my "Gun" notebook.
I can then access Evernote from any internet connected computer, my phone, tablet....almost anywhere. If my house were to burn down, my records would not be lost.
It may not be the best way to document the collection, but it works pretty well for me.
I also keep my wish list in the "Gun" notebook.