First, a caveat. I'm not an expert on the subject and I expect to be corrected by those who know more than I.
Ok good!
From what I've read, if you had a 16" or longer barreled AR upper that was attached to a rifle lower, this is the ONLY size of an upper from which you can remove the rifle lower and put on a pistol buffer w/o having to perform any special actions or pay any special fees to satisfy the ATF.
Eh...? What? Dang that's hard on the ol' brain to decipher.
If you have an AR lower that hasn't been anything yet, you can build it into a "handgun" with a barrel of whatever length, and no stock. (Any buffer tube is ok.)
You can build it into an "Other Firearm" if it has no stock and a barrel long enough to get the whole package to exceed 26" in length. Then you can also use a vertical fore grip if you want.
You can build it into a "Rifle" if it has a stock. If it has a buttstock, you need to make the barrel over 16" long. (Or first register it as a short-barreled rifle.)
IF you'd like to be able to swap back and forth between configurations, make it into a handguns FIRST. If you make it into a rifle first, it needs to stay a rifle unless you register it as an NFA Firearm.
Now, with that firmly in mind, any AR barrel under 16" in length, that began its life as a short-barreled rifle (SBR), MUST ALWAYS STAY configured as an SBR & just the way it is now.
No. That's not right. If you've registered this as an NFA "SBR" then you actually can change it into any of the configurations you'd like. Making it look like a "handgun" won't remove its status as an NFA item (it is still a "Firearm Made from a Rifle") but making it into a 26+" rifle with a barrel over 16" will turn it into a regular "GCA" rifle and effectively removes it from NFA classification.
Any subsequent changes MAY require getting another $200 stamp and doing a new set of paperwork and procedures, etc. (like getting the signature and approval of your local Chief Law Enforcement Officer, aka CLEO) the background check, fingerprints, filling out the Form 1, etc. I think these requirements vary by state.
NOPE.
Here is where I plead mea culpa in not knowing more about modifying an already approved SBR (like changing the length of the barrel).
Rather obviously, which begs the question of exactly why you'd share so much information you haven't done the research on.
HOWEVER, there are ways of doing the conversion from rifle to SBR or pistol that are easier in that direction than going the other way. ...
There's nothing simple about converting any rifle into a handgun or SBR configuration. Either of those gets that gun into National Firearms Act territory and you either need to get your paperwork in order FIRST, or stay well clear of it entirely.
If you have a pistol with a 16" barrel upper - it is no different than having a rifle with a 16" barrel. You can slap on a Magpul stock or stock of your choice and make it a rifle with no need to do anything.
Technically it is a LITTLE different. A rifled-barrel firearm that's over 26" long but has no stock is an "Other Firearm." Not a Long Gun, not a Handgun. And the restrictions on how it transfers are similar to that of a Handgun, not a Rifle. Aside from that, yes, you can make it into a rifle without NFA paperwork by adding a buttstock -- SO LONG AS the barrel is over 16" long.
("Other Firearms" don't actually have to have 16"+ barrels, but they DO have to exceed 26" in overall length.)
Since it is the lower that carries the serial number, it is the part of the rifle that is actually registered.
Common misconception. Regular handguns, rifles, and shotguns -- that are not NFA "Firearms" like SBRs, SBSs, Machine Guns, etc. -- are not "registered" except in a very few states.
This is the "gun" part of the whole enchilada that is sold in various stages of completion - including the "bare bones" format that has no serial number and needs to be further manufactured by the owner.
Wait...what? Hold on. Now you've got "80% lowers" tied up in this? That's just confusing this already muddled mess, but yeah, ok, you can buy some firearms receivers in incomplete condition that are not actually "firearms" yet in the legal sense.
But, that's another story. If interested, just Google "ghost gun."
Why google the anti-gunners' new slanderous term for it? Might as well google "baby killer gun," eh?
If you are converting a pistol or rifle into an SBR, you are, in effect, creating a new firearm and you are required by law to file Form 1 with the ATF and pay the $200 stamp fee.
YES! This is true! Hooray!
The link above explains how you can do all of this online BUT going it all alone carries with it some of the same caveats that used to be said about people acting as their own defense attorney.
They used to say, "He who chooses to defend himself has an idiot for a lawyer."
WHAT? Holy cow, that's just ignorant. It is very simple to file the form 5320.1 ("Form 1") to "make and register" an SBR or SBS. It is pretty straightforward and not really fraught with troubles. Get your paperwork approved before you start making or cutting, and you're A-OK.
That maxim predates the Internet by a few centuries. There are lawyers online who will do everything for you, from simply reviewing your self-made application to doing all of the hard lifting, if you decide to do it yourself, with or without a trust.
I've never heard of a single person asking a lawyer to file a Form 1 for them. To create a Trust? Sure. To file a Form 1? C'mon!
I just bought a pistol lower that I will probably sell by itself OR as an SB pistol that does not require anything more than any other firearm transaction done through an FFL dealer.
Wait. Why are you selling a lower you just bought, or selling a lower you're building into a "SB pistol?" Not sure I see where you're going with that.
If I choose to add a supportive device (like a SIG brace or something similar) I have not changed its pistol designation since the ATF has specifically ruled that arm braces - even if held under the shoulder - are perfectly Kosher.
No, that information is about two years out of date. It is NOT "kosher" to use a SIG (or other) "arm brace" to shoulder an AR "handgun."
Do more research ... or come bearing questions rather than advice ... before you post a lot of information again.
We'd thank you kindly.