Like many others I have turned a walk in closet into my armory/reloading room.
I have a regular door lock on it. Since the door open outwards that makes it slightly more difficult to break into it. But as you already said it only keeps honest people honest.
I'm really fortunate because it is the only closet that also happens to have a ventilation duct for heating and cooling. That helps with the powder storage.
"Socialized Medicine is the Keystone to the Arch of the Socialist State.” -Vladimir Lenin
"I predict future happiness for Americans if they can prevent the government from wasting the labors of the people under the pretense of taking care of them." - Thomas Jefferson (An early warning to Obama care)