Guys, I have searched, but could not find the answer to my question. I currently own an AR-15 that I purchased from an FFL. I would like to build my own at some point in the future and I am trying to be clear on the laws here in our great state. I would like to build something for longer range than the one i currently own and my question is about the muzzle device. If you are running a barrel longer than 16 inches, do you still have to have it permanently attached? From what I have read, it seems the only reason to have this done is for barrel length to be in accordance with the law. Again, sorry if this has been covered.

Yeap it's been covered, and YES you still have to have it permanently attached in NJ.

Federal law requires a 16" minimum barrel length. Nj has an assault weapons ban, and with a detachable mag and pistol grip, you can't also have the threaded barrel. So even over 16", you need a crowned barrel, or something that covers the threads and is permanently attached.

You can't have a threaded barrel that you can unthread a muzzle device.  If you have a 16+" crown barrel and want to attach a muzzle device with a set screw, that is legal. 

As has been said you can have a plain crowned barrel and no muzzle device whatsoever.