I suspect the commissioner is talking about run-of-the-mill full auto weapons with high angle mounts and maybe a little bit of relevant practice for the officers who use them. Not all AA weapons use explosives, the 20mm Phalanx gun system is an example and there are, of course, similar options for other weapons of this caliber. NYPD could also have .50 caliber machine guns for all I know. This has been a standard light AA gun since it was invented in 1916 and there are a fair number of them in civilian hands.

The chances of bringing down a plane, especially a large one, with these kinds of weapons (especially smaller ones) almost don’t exist. .50 calibers were effective in World War 2 because there were a hell of a lot of them and they were after incremental attrition rather than a guaranteed kill. In my experience, civilians who have these weapons, or want them, vastly over-estimate what they will do. That includes the cops. I know a cop who is completely convinced that .50 BMG rounds will penetrate a tank or knock an APC on its side. I hope he doesn’t get to put this to the test.

As for smaller police “special weapons,” a full auto AR-15 is awesome at the local shooting range but it is small potatoes by military standards. A lot of these characters might be influenced by World War 2 movies in which hard-bitten infantry routinely shoot down hostile planes with their submachine guns and light support weapons. It happened more than a few times in real life, but the actual odds of bagging a plane before it bagged you were very poor indeed.