Assuming you mean this device: https://mfiap.com/i-7788127-mfi-fn-ps90-m4-belgium-marked-fake-silencer-barrel-shroud-price-includes-usps-priority-mail-insurance.html
The description says this:
This item slides over your existing 16" barrel and hides the barrel and muzzle device. It is held in place by 4 set screws towards the rear of the barrel shroud.
It does not screw onto threads on the barrel so has nothing to do with the threaded barrel legal restrictions.
N.J.A.C. 13:54-1.2
- Any firearm manufactured under any designation, which is substantially identical to any of the firearms listed in paragraph 1 above. As used in this definition, the term “substantial” means pertaining to the substance, matter, material or essence of a thing and the term “identical” means exactly the same. Hence, a firearm is substantially identical to another only if it is identical in all material, essential respects. A firearm is not substantially identical to a listed assault firearm unless it is identical except for differences that do not alter the essential nature of the firearm.
The following are examples of manufacturer changes that do not alter the essential nature of the firearm: the name or designation of the firearm; the color of the firearm; the material used to make the barrel or stock of the firearm; the material used to make a pistol grip; and a modification of a pistol grip. This is not an exclusive list. A semi-automatic firearm should be considered to be “substantially identical,” that is, identical in all material respects, to a named assault weapon if it meets the below listed criteria:
i. A semi-automatic rifle that has the ability to accept a detachable magazine and has at least two of the following:
(1) A folding or telescoping stock;
(2) A pistol grip that protrudes conspicuously beneath the action of the weapon;
(3) A bayonet mount;
(4) A flash suppressor or threaded barrel designed to accommodate a flash suppressor; and
(5) A grenade launcher;
Note that the admin code says you cannot have 2 of the prohibited features. Let’s count what you have with a PS90.
(1) A folding or telescoping stock; - nope. Fixed stock
(2) A pistol grip that protrudes conspicuously beneath the action of the weapon; - nope, no pistol grip at all
(3) A bayonet mount; - still nope, not there
(4) A flash suppressor or threaded barrel designed to accommodate a flash suppressor; - yep, got one of those
(5) A grenade launcher; - nope
So even if the FFL is correct in thinking that this device counts under #4, it doesn’t matter because none of the other evil features are on this type of rifle. Your evil feature count is 1.
Is the flash hider that came with the rifle pinned? It doesn’t need to be.