I'm sure you've seen the huge filing-arrangements where shelves can be cranked together so they completely fill a room, with just enough empty space between to allow someone to walk between 2 rows? All the shelves are moved aside until one gets the opening to the row they need...

Now - imagine something similar inside the safe, except in this case it's running front-to-back instead of side-to-side... This would allow packing a bunch of rifles into a small space. To get them out, one pulls all the racks forward of the target-row out until he can reach the opening containing the rifle he seeks.
This would require building frames (from, say 2x4 stock) with slots for the butts and some sort of retention for the top to keep the rifles from tipping over. Something as simple as a hook-and-eye closing each slot would do...
With such a method, it would seem to me one could fill a space with minimal distance between the rifles and thus get more into your safe than you'd imagine...
Alternatively, they could be mounted on the sides using bearing-slides like are commonly used in server racks in a data center. Each rack mounted to its own set of slides on the side-walls...
This is clear in my head - I hope it makes sense here...
DD