Ahh yes, forewarn your opposition by telling them "I have a gun".

A bad guy knows he's bad. He knows breaking in your door is against the law. Bad things happen in the breaking and entering trade. Getting shot from ambush by a homeowner's 12 gauge shotgun is one of those bad things. I see no compelling reason to give any advantage to a bad guy by telling him where I am or that I'm armed. That will become very clear shortly after the door fails.