Or an 80% lower requires no paperwork at all
There are some in polymer if you don't have heavy duty tools
I am not condoning 80% just clarifying since your new to ARs
To clarify; If I buy a lower housing from a manufacturer and a lower parts kit from a different manufacturer then get all the rest of the parts eg.upper, barrel, gas block, etc. I would have a working AR with no paperwork involved? What am I missing?
No. If you buy a lower receiver (stripped or complete) from anyone other than an individual, i.e. you buy it from a manufacturer, online or local gun store) you complete the NICS form just like any other firearm. You buy a lower receiver from an individual, i.e. from the classifieds here, you do nothing, just like any other firearm.
The one odd ball is if you buy an 80% lower which requires you to mill out the various pin holes, magazine well, etc. Then you are the manufacturer, and no forms required. It does require an amount of tooling and know-how, however.
What may be tripping you up is terminology. Lowers are usually called either stripped or complete (ignoring 80% lowers for this discussion). Complete has the various pins, fire control group, grip, stock, etc. installed so that it is ready to slap an upper on and go to the range. A stripped lower is just the physical lower and you install the various pins, fire control group, grip, stock, etc.