Years ago I helped a friend in Stillwater OK build a gun room. He had an extensive collection of long guns, mostly suppository, but about 25 were ML'ers.
The room, as I recall, was about 20' x 40'. We built the walls first, a double concrete block wall. The vertical holes had 1/2" rebar, 3 pieces each, extending into the foundation, then filled with concrete. The ceiling was monolithic concrete pour adequately rebarred. And the only door was an ex-bank vault door.
He had a umber
It was about the most secure "gun room" I've seen. And my pay was a VW-powered dune buggy that I used throughout an Oklahoma winter to get to my classes at OSU, and my work testing soil samples for the state. Good trade!