You have a lot of good information above.
Let me add to that.
Let's suppose 30 long guns at only $500 each, that's $15,000 worth of arms. Perhaps spending a bit more than 1/15th that value to protect them is apt.
For my 2¢, I'd actually be looking at two (2) "20 gun" RSC for that many arms. One is physics, smaller cubic footage is easier to fire protect than larger. Also, you buy steel by the pound--takes a lot more pounds for a 40-45 gun safe than a 20. But, mostly to get better access.
See, a 40 gun RSC is going to have to "stack" arms to stow that many. Unpacking 10 or 15 arms just to get to the back row is a pain (shoot, I don't like having to pull 4 arms out to get to a 5th).
Now, really, given how many arms you want to protect, I'd actually look at "hardening" a closet (4x16x8 concrete block over roxul batts) instead of an RSC. Armoring up a 36x48x96 closet is way more space than even a good UL safe, and has no "look at me, I'm full of valuables" to it at all. Ok, so that could run to two or three grand--but isn't your 15 grand of goodies worth it?