I've been reading through the pinned and open posts and am still a bit confused so I wish to be educated completely before I make any moves. I've been thinking about an AR pistol not that I live in the "Free" State on Tennessee. I have a 80% lower to complete. If I buy a complete upper with a short barrel do I need to register it?
I read that if the lower has never been registered as a rifle then a Form 1 is not required. But do I need to serialize the lower and bring it to an FFL to register, or would I be good as is?
I just want to make sure I am legal if I go forward with this build.

If you are home building and the receiver has never been a firearm then it’s whatever you want it to be.

Just start it as a home built handgun following your state's and the federal laws on the topic (which should be basically don't be a prohibited person for people in free states) and never configure it as a rifle or short barrel rifle. Remember the rules on VFG's and you're pretty well good to go. You won't need to register the upper anyway and registration of the lower is according to whatever your state's laws are. Basically, you're GTG with a 80% lower and a short barrel upper.

There is no federal requirement to register any gun.  You will need to see if TN requires any registration of firearms.  If you put a rifle stock on a firearm with a barrel length of less than 16" you have created an SBR and must file paperwork as such.  If you put a pistol receiver extension on it, with or without a brace, it is a pistol and therefore unrestriced (federally).  Again you need to check your state reg's to see if they require registration of handguns.  An 80% lower, milled by you, can be whatever you want it to be and again unless your state requires you to register it, is unrestricted.  Build your pistol and be happy!

Thanks Guys, Looking into it further for Tennessee. I've got my eye on a BRN-180S in 300AAC with the 10" barrel.

Ok, just for clarification, I'm reading that the main difference between SBR and pistol is the shoulder stock. One of the builds I am planning would have a 10" barrel and a folding stock. This one using the BRN upper can be fired with the stock folded. Does this make it a SBR and would it require the stamp.

Rifles have stocks, pistols do not.

A pistol can have a brace.
A rifle can have a brace
But only a rifle can have a stock

URGs are only looked at for the barrel length when you have constructive intent or it is installed on a lower and are not otherwise regulated.

The folding has nothing to do with it unless your state has a restriction on OAL. (over all length)

It sounds like you are new to all of this.
The ATF doesn't make things easy as they constantly change their minds.
Read up on NFA law in regards to SBRs and pistol definitions and then reference your state statutes.

Happy hunting.

If the barrel is less than 16" and it has a shoulder stock it is an SBR, regardless if the stock folds or not, and subject to the $200 tax.

FIRST COMPLETE build is what the firearm becomes, not what is attached to the butt end of the lower.

If you put any receiver extension on a new lower, complete lower but no upper, it is nothing until you put on an upper, it's still an "other."

Let's walk this through.

You have a completed lower with receiver extension, doesn't matter what kind RE it is.  You have no upper attached.  Your lower is an OTHER.

You put a stock on that receiver extension.  No upper.  It is still an "other."
You put a brace on that receiver extension.  No upper, it is still an "other."

You take off the brace or stock.  ***Remember this next step because it determines what your lower will be from this point on.***

You put on any length upper, nothing on the receiver extension, no stock or brace.  It doesn't matter FEDERALLY if it is a 7 inch upper or a 20 inch upper, it is NOW a completed and fully functional PISTOL and will be a pistol from this point on.

If you add a brace to it, no matter the barrel length, it is still a PISTOL, nothing changes.

Now you remove the brace and put a stock on the above PISTOL.  Here it starts to get confusing.

If the barrel on the above "pistol" is 16 inches or longer, it becomes a RIFLE so long as the stock is on there.
But, if the barrel is less than 16 inches, you have created an illegal SBR because you put the stock on it.  If you had put a brace on there instead of a stock, then that's fine, it's still a pistol.  But a stock, any kind of stock, telescoping, fixed, or folding, makes it an illegal SBR.

Now, let's go back a bit.  You have your completed lower, with receiver extension, no stock, no brace, and no upper with any length barrel ever being attached.  It has never been completed as a fully functional firearm, it is still in the "other" category.

You add a stock to the lower (remember this step).  Lower is still an "other" at this stage.  Now you add an upper with a barrel 16 inches or longer.  You have now a completed and fully functional RIFLE and will be a RIFLE from this point on unless you SBR it because the first completed build was a RIFLE.  

Notice the order of things.  You attach any length barreled upper to your lower first with NO stock on the lower, it is a pistol.  First completed build was a pistol, it is now a pistol.

But if you attach the stock first, leaving it on, then an upper with a 16 inches or longer barrel, it is a rifle.  First build was a Rifle, and now it is a rifle forever unless SBRed.

Again:
if first completed build was a pistol, you can swap back and forth from pistol to rifle to pistol as much as you want so long as you never have a stock on there (and it doesn't matter if it folds, telescopes, or is fixed) with an upper that has a barrel length shorter than 16 inches.  The stock can only be on there with an upper that has a barrel 16 inches or longer.

But if the first completed build was a RIFLE, you can never make it a pistol (barrel length less than 16 inches) or it will be an illegal SBR, unless of course you SBR it before doing so.

To make a long story short, the last thing you should ever add to your build, even if the barrel is 16 inches or longer, is the stock.  That makes all your lowers PISTOLs, at least in the eyes of the Feds.

The first completed build is what it is, not just if the lower had a stock or brace or bare receiver extension attached to it.  Without ever having an upper on it, no matter what is on the back end of it, the lower is still just an "other."

Stupid?  No question about it, but that's your tax dollars at work for you (or against you).
 

As surmised, yes new to this when it comes to the nuances of AR platform pistols and SBR's.
But this has been very helpful and informative. Now, since i will be using 80% lowers that have not had anything on them I will first configure them as pistols, once that is done I can add the folding stock to the BRN and still be legal even with the 10" barrel. Typical bureaucratic confusion, thanks for making it a bit more clear. I have everything available to complete these builds, I finished milling the lowers the other night so it's just a matter of assembly.
first and foremost I want to stay as clean as I can (legally).
Thanks again!!

Ok, if that's what I need to do, so be it.... the $200 stamp kind of defeats the purpose of having a 80% lower, although from the sounds of things they are going to need to be either registered or destroyed if the libtards get their way.