Since the late 1800's (1890 I think - the Sullivan law) ... they also had a law that stated "use a gun, go to jail" and it's the first thing they (the DA) plea bargins away ....
Actually it was passed in 1911 after a failed assassination attempt on the city's mayor. IIRC Sullivan was a police commissioner who drew up the law and got it passed.
According to an article I read a few years ago in a gun magazine, the real reason Sullivan got the law passed was that he was paid off to get it done by a group of Irish dockworkers whose jobs were being taken by recent immigrants from southern Europe who would work for less pay. It seems the Irish dockworkers were beating and often killing the immigrant workers to intimidate them, hoping it would scare them away from working the docks. And their Irish cop friends in the area were ignoring the violence and actually doing some of it themselves.
After a while the Italian workers got tired of the abuse and bought guns to fight back. After several Irish thugs were shot and killed, the Irish workers and their Irish cop pals demanded that Sullivan get a law passed to disarm the immigrants. When the attempt to assassinate the mayor with a revolver was given wide publicity, Sullivan had the excuse he needed to get his namesake law passed.
I don't remember what gun rag that story came from, and I'm not certain that all of it is true. But having some knowledge of the corruption that was so prevalent in NYC government at that time (when was it not?), and the blatant favoritism that Irish NYC officials and cops gave to their Irish friends and families in that era, I tend to believe that most of it is true.
Note: I'm 1/2 Irish myself, so don't accuse me of Celt-bashing.