Blast dissipators/diverters/diffusers are not considered flash hiders in NJ.
The pinky test is not a real test.
In any event, all flash hiders are legal in NJ in and of themselves.
They are merely one of the 6 features on the “evil” list that defines “Assault Weapons” in NJ.
You are permitted to install two of the “evil” features on a Semi-Auto rifle and still be legal in NJ.
A flash hider does not become illegal until it is the third “evil” feature on a semi-auto. And even then, it isn’t the muzzle device that is illegal, it’s the entire firearm.
• You can put flashiders on bolt guns.
• You can put a flash hider on a Mini14 or Fight Light SCR with no issues (the flash hider replaces the non-existent pistol grip for your second evil feature).
Since an “OTHER” or SBF is defined as a firearm and decidedly not a rifle, the Assault Weapon restrictions/definitions do not apply.
You do have to follow the federal definitions to avoid inadvertently building an AOW or SBR:
SBF Requirements:
• Over 26” in length (measured from the back of the receiver extension with the brace off to the end of the unpinned barrel or pinned muzzle device)
• It can not have a stock (can have a brace or a naked receiver extension)
• It must have a VFG (AFG types are not acceptable)
• The lower must have been transferred as an “other” not as a complete rifle, and have never had a stock installed.
Some manufacturers are engraving “SBF” or “Other” on the lower as part of the serial or model number as a marketing tool. It’s a nice feature but is not a necessary step to possess a legal SBF.
If you were in a state (definitely not NJ) that permitted Short Barreled Rifles/Shotguns (SBR/SBS) and filed the appropriate NFA paperwork (Form 1) with the Feds to convert a rifle or shotgun into an SBR/SBS then you would have to engrave the lower accordingly - But you do not have to do that for an SBF.
Finally - And this is important -
GUN STORE EMPLOYEES ARE NOT PAID TO GIVE YOU LEGAL ADVICE!!
I repeat -
GUN STORE EMPLOYEES ARE NOT PAID TO GIVE YOU LEGAL ADVICE!!
Do not rely on gun counter salespeople to keep you out of jail, or to sell you things that you can technically have but the employee doesn’t think so.
They are paid to sell guns and keep the shop out of trouble. They are not required to have any training or certification in NJ or Federal laws past what is required to take your money, get you through NICS, and get you out the door.
Take legal advice from lawyers*.
* Yes, that includes advice on Gun Forums - including what I wrote above. I am not a lawyer. Accept what I said above at your own risk.
If you don’t know, research what you learned and vet what you hear/read here and elsewhere.