I would never announce that I was armed regardless of whether in my home or in a public place. There are two reasons for my position. First, is that it (IMO) is a tactical error. I learned in Nam that surprise is a big advantage in battle. And from experience I can say it is a BIG advantage. Second is what an excellent criminal defense lawyer told me 20 years ago when I paid him for a 2 hour consultation. The following summarizes what he told me as best I remember it. It is not legal advice because I am not a lawyer and write this only as my experience and point of view.
Under PA law (and many if not all states’ laws) you can only use deadly force when you or a person you defend is threatened with death, serious bodily injury, or rape. You cannot point a firearm at anyone unless that have a visible deadly weapon or there’s is a disparity of force (an ambiguous term). Displaying a gun in any other situation is criminal brandishing, and an aggressive prosecutor will charge you for it. Of course, announcing being armed is not brandishing, but by an aggressive prosecutor might considered it provocative or a threat and use it against you even if you do not use the gun. Threatening people is often seen as assault, which is prosecutable. If you feel under threat of harm announcing you are a a potential threat is not good judgement. It only puts you adversary on alert and could work to you disadvantage legally and tactically.
The above applies to outside your home, and the home is defined is may state laws. In PA my house, place of business, car, any residence I might be occupying and a hotel room I am in is considered my home. Back when I met the lawyer he advise me unless I saw a deadly weapon on the person invading my “home” I should announce I was armed and give the invader a warning and the opportunity to retreat. He said the same rules about deadly force applied to an in-home encounter as applied to a public encounter except if they would be not likely be applied. However, an aggressive prosecutor might charge you for using the gun if the invader did not retreat. In PA you cannot use deadly force to stop property crime; not even a car-jacking unless the perp enters your car while you are in it. The lawyer told me you could warn a home invader that you were armed. In that case it would not likely be seen as provocation or a threat, and only be seen as a defensive warning.
Since that time Pa codified a a Castle Doctrine which bars criminal or civil legal action against a person who uses deadly force against a “home” invader. The world home is defined in the law, and it extends beyond the interior of the house. So considering the PA law I thought about what I would do is someone invade my “home.” First no warning of any kind including having a gun would be given,. In most cases they would know I had a gun because it would be immediately visible and pointed at them. If they retreated I would not fire because the immediate threat t0 me was gone. If they did not immediately retreat I would shoot. Why shoot immediately? I a house the invader is going to be very close and could close the distance in a less than second or two. In a car the perp is next to you. A warning would a tactical error. Never give an adversary a chance to get closer to you and a warning could work against you. However, if id did not see a weapon I would this “leave or die”, and if the took a step toward me or fails to retreat I would shoot.
Every gun owner ought to understand their state law when it comes to self defense in public or in the home. In PA there is no mention of giving a warning to anyone, but it is clear when deadly force can be used.