In order to be legal, the magazine must have been manufactured before September of 1994, prior to the passage of the Clinton ban. In order for the rifle itself to be legal, it may not have two or more of the following cosmetic features, unless it was manufactured prior to September of 1994 (the M&P15 was not in production at that time):
When the 1994 so-called "assault weapons ban" went into sunset in 1994, some states wrote the ban into their state legislature, to include New York, New Jersey, Massachusetts, Connecticut, and Cook County, Illinois (California's ban on such rifles predates the Clinton ban, being written into the state legislature in 1989).
If you currently own such a rifle, and are considering (or in the process of) a move to NY, it's very easy to make it NY-legal. You can have ONE of the above listed features on a detachable magazine centrefire rifle in NY. If your move to NY is not going to be a permanent thing, you may want to keep your flash suppressor, unless you don't mind permanently pinning and welding a muzzle brake on in its place. You can do the following:
No.
No
Depends entirely on the magazine. Some are not meant to be taken apart.
Yes
Drop the magazine and clear the chamber.
Lever Action Magazine Pistol
Smith & Wesson semi-automatic rifle
81-85
100-500 USD
22 long rifle
Started in 1983
Impossible to answer without a detailed description.