If, as you say you will be securely fitting a gun cabinet, that is fine. I would only question why you feel there is a need to fit that lock to the outside of the cupboard, reason being, if someone broke into your house that lock on the outside would in fact attract them to get the jemmy out to investigate further.
Just my 2p worth
yep it will defiantly be fitted securely to a solid brick wall. Just thought it would be a good idea to put a lock on it just incase.