If this is not concrete construction in walls floor and ceiling, have your contractor insert rebar rods horizontally through a separate set of upright wall studs at about 6" intervals around all walls and ceiling, and the floor if possible. Something that will stop a chain saw and keep people out long enough to discourage them.
Make sure it has electric and if possible heat & AC for climate control. If possible make it at least half again bigger than you planned, because you will regret it later.
Mine is 4"X7" interior, and is safe only, located within my "Gun Room", which has work counters around 3 walls for loading press's, cleaning/repair, and shelving for gun cases etc. The "Gun Room has separate steel door to go through before getting to the vault room. Don't forget plenty of electric outlets at bench top height and your TV and radio shelving. Include plenty of shelving for Tools, dies, powder, primers, etc. Leave space for under bench storage of ammo.
