Anyone walking along a Brooklyn street in possession of a visible firearm is almost certain to draw the attention of the police. If police determine that the weapon is lawfully in that person's possession, how far they can go in searching or detaining the individual without evidence that the person is committing a crime could violate someone's constitutional rights.
Just how far the Second Amendment to the U.S. Constitution goes in affording people the right to openly carry a gun in public without facing weapons charges for violating New York laws is a complex legal issue. What appears to be clear, however, is that once police determine that the weapon is lawfully in someone's possession, the Fourth Amendment limitations on searches and seizures come into play.
Absent probable cause on the part of the police to believe that a person is committing a crime, the Fourth Amendment does not give them the right to detain and search that individual. Observing that a man has a gun might allow a police officer to approach and question him to verify that the weapons possession is legal, but once that is done, detaining or searching him could violate his constitutional rights.
Arrests based upon weapons crimes may begin as brief encounters on the street. Police may stop someone to ask questions about activities they believe to be of a criminal nature. These encounters can quickly escalate into searches and seizures that could provide evidence of felony gun charges and, in some cases, additional charges of robbery or murder. Getting legal advice from a criminal defense attorney about one's constitutional rights before speaking to police might prevent the loss or waiver of important rights.