They gave a year to sell, destroy or register the weapons. Now they did make the sale/transfer within NY immediately prohibited.
However, pretty simple solution to that really...
“No, officer I no longer possess the weapon, I sold it out of my car truck to a Vermonter when I was at my buddies camp in Vermont. The leaves were turn, it was beautiful that weekend. Have you been to Vermont.”
-or-
“Hey, I left it at my buddies camp in PA.”
Point being it’s not illegal for a New York State resident to transfer and/or possess an “assault weapon” outside of New York and the enforcement arm within New York has no jurisdiction in any of these places. (In other words “piss off”.)
http://troopers.ny.gov/Firearms/PPB-12.pdf?OWASP_CSRFTOKEN=YJXI-QWII-HL1J-48KM-GQCB-UQ2D-WBKG-MR68
Not so simple to claim you made a private sale to a VT camping buddy...