The big difference between Ft Knox and other safes or vaults is that they use concrete in between the sheets of steel that make up the panels and door. Others use wallboard to fill that space, and add some fire resistance, but it doesn't come close to the insulative properties of concrete in that application. Yes, it takes a lot of people to move one of those heavy bastards, too. No way I'd put that on a joist floor unless you double 2x12's with posts underneath. They're almost impractical for homes unless you have a slab, or a walk-out basement.

My final solution is going to be a vault door. I have a huge walk-in cedar closet in the central part of my house that I am going to lift the boards and line the studs with pig wire and call it good, other than insurance. between a heavy steel door and walls that are almost impossible to breach, it should qualify for my FFL inspection.