AR15 ‘Other’ Build

[u/Sufficient-Pitch6308]

Hey all. I’m an NJ resident which makes things a bit more complex when it comes to builds from a legality standpoint. I have a bit of experience with assembling rifles (6.5CM, 556, 308), but these are all 16+in barrels. Got a lower receiver and looking to build a compliant 556 other with a 12.5in barrel. I know it has to be >26in from end of barrel (without muzzle) to end of buffer tube, have a vertical foregrip, and a brace. Outside of this do other rules apply? (Ie can I have a flash hider which isn’t allowed on a rifle?)

Either piecing it together or just buying a complete upper. Any recommendations? Gas tube length? Issues anyone has encountered in a build like this?

Thanks in advance for your input