Aside from the creative storage methods mentioned above (especially the very effective muzzle up/down technique), I'll let you in on a little known secret.
It doesn't matter what the manufacturer says the safe will hold. Two safes, from two different manufacturers, with the same internal dimensions, will hold the same amount of guns.
All of the manufacturers install their interiors, count the slots, and claim that as the capacity. The problem is that 1 slot won't always hold one gun. However, 10 cubic feet will always hold 10 cubic feet. When comparing capacities, disregard the number of slots, and look at the internal volume.