In NJ we play the "evil features" game - you can have 1 evil feature on a semi auto rifle w/ detachable mag. On ARs that one is the Pistol grip. Any stock your cousin-in-law will get will need to be pinned, or use a fixed stock (like a Magpul Fixed Carbine stock). Bayonet lug (if using an A2 front site/gas block) will need to be ground off, and a muzzle brake/compensator will have to be pinned/welded on. A threaded barrel is one of the evil features.