This probly the best one, make sure when you paint it to take a sample of the paint already there to homedepot or somewhere that mixes paint so they can match the exact color of the wall too, otherwise u gotta buy more paint to paint the whole wall, or it wont match.
I'm a General Contractor, so it may be easier for me....
But for something that you want hidden, but have access to if needed (Gun)...just cut a square in your drywall somewhere (Maybe 12 inches by 12 inches, or bigger for a rifle), put in a piece of wood as a shelf, then put the drywall back, but don't use any backing on it-- Just tape it, mud it (Must be a perfect job), and paint the wall-- So when you need it, it won't take much to hit it, and grab what you need--
Like I said tho, I have a lot of experience in drywall, if you have textured walls, it might be hard to do-- I can match almost any texture, but most can't--