FYI
Per request, SST's Rifle Room has been updated with a new section with links to 21 gun safe manufacturers.

I have personally observed this:

No matter what size gun safe you get (or have built) it will be too small.

I didn't believe it at first, but I do now. They don't shrink because I have measured them before and after. They are simply too small -- always.
I think it must be a design flaw common to all safe manufacturers.