Henry,
I checked back into the laws for you and saw in the State Laws and Published Ordinances - Firearms 2005 26th Edition, pg.305 Under "Ammunition-Feeding Device - Magazines, belts, feedstrips, drums or clips capable of being attached to or utilized with any center-fire rifle, shotgun or pistol which employs the force of the expanding gases from a discharging cartridge to chamber a fresh round after each single pull of the trigger which, in the case of a rifle or shotgun, holds in excess of five cartridges, or in the case of a pistol holds in excess of 17 cartridges."
I also noticed that some other major websites refuse to ship mags to NY. As a new FFL we are being extra cautious when it comes to state laws, if we find information that contradicts this we would be glad to update our website.
Mike