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In what way does it become a non compliant firearm assuming I follow all of laws like 16" barrel and no muzzle device? I dont know so im asking

Yes it would. What I mean is that if I made the upper from I go to the hardware store and make an upper receiver from metal tubing with a 16" barrel, would it be legal?

I figured the PRNJ had a technicality were they could get for you manufacturing if you made youre own upper

since the upper receiver is not a serialized part and not considered a firearm, would it be legal for me to say, go to the hardware store and buy the right materials to make an ar upper that would work EDIT: Clarification: What I mean is that if I made the upper from I go to the hardware store and make an upper receiver from metal tubing with a 16" barrel, would it be legal?