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Don't fuck about with hinges. Just use a lift out door, cut out of the side, that has a lip holding it at the sides and bottom and the bolt of the lock holding it at the top.

One here in Christchurch:

Endorsed Weapons Safe (A/B/C/E Category) | Trade Me

Baronet Safes:

Baronet – TSL and Select Engineering

They should meet the criteria. There's one in the corner opposite my local Arms Officer's desk where they keep the station Bushmasters...