I was recently in the home of a collector with something similar to what you describe. He'd added the vault to his ground floor (the home was a split level) when he built an addition. It was fully finished, so I can't comment on the construction materials, had a security gate akin to those on retail establishments, which was concealed behind two normal french doors. Two walls of the room were lined with rifle racks, with the long guns standing vertically. The rear wall had two revolving standing rifle racks. One front wall had a glass enclosed set of shelves, probably out of a retail store somewhere, on which handguns were stored. I'd estimate the room was about 12'x15' - it was big enough for a pool table in the center - and there were well over 100 guns displayed. It was quite an impressive setup.
