"This really isn't complicated. You should be able to buy a receiver and create a firearm from parts and that legally purchased receiver, because legally, the numbered receiver is already a firearm. You are adding unregulated parts to it."
You would think that to be true everywhere, using common sense and some knowlage of "The Law", the problem being that there are 52 sets of laws to deal with: Federal, 50 states, plus DC. Probably more if you want to deal with US Posessions as well.
Within the context of FEDERAL law it's true.
It is NOT TRUE according to the STATE law of New Jersey.
In NJ, you must buy the receiver as if it were a firearm, but then *as a second set of circumstances* if you then activate it to be operable you have "Manufactured" a firearm under NJ law. Do that without a NEW JERSEY license to manufacture (not a FEDERAL license, and one not available to individuals) and you are a felon.
Here's another NJ kicker as well: You need a Firearms ID card to buy a long gun in NJ. You need an additional pistol permit for EACH handgun you purchase in NJ. Since an AR-15 lower receiver *sold as an individual item* is neither a long gun or a handgun, and can be built into either, what permits are required to purchase one in NJ? Under Federal Law it's sold as an "Other" and you can build it up either way. Under NJ Law.... Hmmm......
OK, never mind about *getting it to begin with*, let's make it easy: In the end that if you build EITHER you are breaking another law regarding manufacture of a "Firearm", so why bother? Trust me here, I've already stood across from a Judge explaining myself in this regard. Thank goodness he was sympathetic. I found out later that he was a shooter, but.... <whew>.
The bottom line is that if you live in NJ, stay away from the popular hobby of building up a nice AR-15 from a lower. Start with a completed rifle, strip it, and rebuild it to suit yourself.
BTW... in NJ if you buy a stripped .45 Colt Frame and build up a custom pistol using it, you've done exactly the same thing: Go directly to jail. I almost ran afoul of that as well.. thank goodness the frame that I had literally bought at the same time (same visit to the same shop) as the AR lower had not yet had it's slide fitted. It was a VERY close thing and would have been a VERY bad deal for old Willie.
So glad to live elsewhere now...