"Out of sight of casual visitors" takes out the option of entrance hall or probably living room. bathrooms are impractical and far too humid. Kitchens could work if large enough, good place to hide a cabinet in a cabinet? If you have other options like under stairs cupboard then that is more practical than bedroom when you are getting out of bed in the small hours.
dont forget that there is nothing stopping you disguising the cabinet as long as it is secured to a solid part of the building.
i once rented a place with all walls being timber frame and plasterboard so useless for decent fixings, I laid the cabinet on its back under the stairs fixed into concrete floor. with a Hoover, boots, coat hanging off a rack etc under there you would never have looked for a gun cabinet!
good way of fitting lots of guns into a cabinet too.