Well I know this has been hashed out here already but I have to say stop spending money on guns and figure out a way to get a larger safe .
Luckily, given this tit for tat trading method I have to adopt, it does mean that a new gun is usually not too expensive, since part of the cost comes from the sale of its predecessor, but that is another point.
I mentioned this earlier, but I live in a flat which is largely loft conversion meaning sloping walls. It is very hard to fit a large cuboid in most of the flat that would not encroach on living space.
There is also the issue that my wife is not enamoured with the idea of a whopping great big steel block in the living room. She supports my hobby even though she actually hates guns and I respect her support but the trade off is that I don't rub it in her face and a 300kg box in the living room might chafe quite a bit!!
I'm not sure of your age but I'm sure your current number of firearms is not going to work for you the rest of your life .
Well, the truth is that all my guns are cases of want, not need. I don't hunt at present. My IPSC is a sporting pursuit and I only ever shoot at the range.
From a SD/HD point of view, I really could just make do with one of my handguns. Arguably that is really the only gun I need.
Don't get me wrong, I'd dearly love to increase my collection but I have to be realistic and at present a bigger safe is not really an option.
It may become possible in the future, but not where we live now.
A note on the safe companies: the company designs a safe and issues a statement saying how many guns are supposed to be in there. That is the statement that the police go buy.
If it is a 20 gun case and you can comfortably fit 25, my guess is it probably isn't an issue. It would need to have certain wall/door thickness specs as well as being fixed to the house's structure.