I have my opinion, but I am only a Leo, not a LEO.
The simple answer is a reference to the question that shows up on the bar exam in the criminal law section: "What crime or crimes have been committed?"; if you can't come up with a specific answer, then the answer is, "none". If they are violating a specific statute in your presence, then you make an arrest. Or if they make threats with reference to the firearms (any use of a firearm in connection with an attempt to intimidate or coerce is "brandishing" in Virginia), or do anything with the gun other than wear it or use it in the reasonable defense of self or others, given a threat of imminent serious bodily injury, then make an arrest.
Otherwise, it's just something they're wearing, like shoes, and ought to be given no more importance than that. You have no duty as a police officer to ensure public safety; the courts have said so over and over again. It is not the job of the State to prevent crimes, but to apprehend and punish those who engage in criminal activity.
Anyone who gets worked up on one side or the other of this issue has lost - don't let Satan steal your peace.