If I was to hide a gun in the closet I'd do it this way.
Get a long mirror for the closet and at the back of the closet cut the sheetrock so, between the 2x4s, you can slip in a shotgun, rifle, or a few handguns, then put hinges so you can mount that mirror over the new gun hide.
Sink the hinges, and mount them on the 2x4s, so the mirror swings open to expose the guns. Use magnets to keep the mirror door closed.
That way, you have a good walk in mirror AND you can keep a long gun real handy.
"The government has confiscated all of our rights and is selling them back to us in the form of permits."