The safes you buy at box stores are not "real" safes. If you're putting big money NFA its in it, you need a REAL safe. If its big enough for a whole bunch of guns and its a really high-end safe, it's going to be 5 figures.
Take, for example, a Fort Knox Legend. (This is not necessarily a recommendation, only an example.). It's EASY to configure on that's over $16,000.
For that money, it may well make sense to convert an extra room or walk-in closet to a vault.