Let me weigh in as a CCW holder who quite frequently carries concealed:
If I were at a party (not that I attend parties frequently) and if I saw someone carrying NOT concealed…my reaction would depend entirely on how well I knew (or thought I knew) that person.
In my state, it is NOT illegal to carry openly, except within certain city limits. However it’s certainly considered bad manners. Like…wearing a coat made of real fur. On certain streets it might well cause a panic - and that is the real concern, and AFAIK that is what the person would be charged under: disturbing the peace. On the east side of the state, a rifle in the back of a pickup truck is neither unusual nor does it panic people. On the west side…
In my state is IS illegal to carry concealed without a CCW. And when they say concealed, they mean concealed: nobody should be able to figure, guess, see, or ascertain that you are in fact carrying a weapon, loaded or otherwise. Nor may you, without a CCW, carry a loaded gun in your vehicle, although you may carry, say, the gun up front, and the ammunition in the trunk.
I have never (to my knowledge) been ‘made’. It’s possible once: I got the visual once-over by a sheriff at a 7-11 while wearing a concealed-carry fanny-pack. But as the officer said nothing, it’s possible he was just looking. Or was not concerned.
But to return to the question: if I were at such a party, and someone at the party were careless enough to allow themselves to be seen to be carrying? If I didn’t know them, I would tell the host I was leaving, and why (since presumably they would know the person). If I did know them and knew them to be stable and peaceable people, I would approach them and tell them I’d seen their piece and they needed to be more careful, maybe talk quietly about CCWs. If I knew them and had any doubts about their stability (for example, if I knew they were easily angered, or if they were drinking), I might well call the police. It all depends. It’s never a matter of gun == okay or gun == ACK! RUN AWAY! for me.

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