Based on how the law is written, it reads as though the muzzle device would have to be a threaded add on too be illegal, but you know i actually take the laws for what they say and don't translate them to mean what they don't say.
Taken from NYSP field guide, curious if its ever been updated.
A semiautomatic rifle that has an ability to accept a detachable magazine and has at least one of the following military characteristics:
a. Folding or Telescoping Stock
b. Protruding Pistol Grip
c. Thumbhole Stock
d. Second Handgrip or Protruding Grip that can be held by non-shooting hand
e. Bayonet Mount f. Flash Suppressor
g. Muzzle Brake (someones corrected the spelling even though this shouldn't even be on the list anymore or did an amendment get passed lumped in with Cuomos latest exemption for politicians corruption bill.)
h. Muzzle Compensator
i. A threaded barrel designed to accommodate the above
j. Grenade Launcher