What I think I know:
1) This was purchased as a 9mm pistol directly from the manufacturer (new).

2) I added the SB01 brace shown.
3) ATF says the pistol is still a pistol UNLESS it is shouldered, in which case it becomes an illegal SBR.
4) If I pay those bastards $200 and wait for the Form 1 tax stamp, I would then have a legal SBR.
5) I could then legally shoulder my SBR.
6) If I ever remove the SB01, the pistol once again becomes a pistol.

If any are NOT true, call them out by number and let me know what is correct.

Now the kicker: If after 6) above, I put the SB01 back on and shoulder it, have I:
a) made a NEW SBR (requiring a 2nd tax stamp to become legal when shouldered)?
or
b) restored my old SBR to its already authorized SBR status?

The reason behind all this is when I carry it inside the house, it's at waist level and I use the green laser to aim (the SB01 just makes it heavier and longer). When I'm outside, I shoulder it and use a red dot (not shown). Inside use is much more frequent, but both inside and outside could be done the same day.

And yes, I know:

Once it's an SBR, it can go from SBR to pistol to rifle to... whatever, and back again.

Taking off the stock doesn't remove it from SBR status. You writing into the BATFE removes it from SBR status.

Doc, your understanding is correct for 1-6, and option b is correct (1 stamp is all you need). SBR status is determined by actual configuration, not registration, although registration remains in effect until you request its removal.

3. The ATF wrote an "open letter" regarding the arm braces. Basically saying when you shoulder the pistol arm brace you are temporarily "reconfiguring" the pistol into a short barrel rifle. I think the ATF only issued the letter because they were tired of people writing/calling them asking if it was legal. If they were seriously about enforcing this you would have known by now. The truth is not one person has been charged for doing so, I wouldn't give it too much thought. If you are happy with the brace, enjoy it. If you want to use a real rifle stock then file for a stamp.

Link to said open letter if you care to read their full opinion in detail.

https://www.atf.gov/file/11816/download

4. Correct.

5. Correct.

6. The fact is even after getting the approved Form 1 SBR stamp your pistol with a SB01 installed is still currently only a pistol. The ATF doesn't care about the "registered" status, they only care about the current configuration or state of the firearm. Does the firearm have a rifle stock installed along with a barrel under 16", then it's currently a SBR. If that same firearm now has a 16" upper installed, it's no longer a SBR and only a title 1 firearm. The SB01 adds a unique situation to this scenario where the ATF interrupts the law to mean once you shoulder that pistol with SB01 you have temporarily created a SBR, but only by the act of shouldering, not by the fact it's stamped.

As for your other questions/scenarios. The stamp is constant until you write the ATF and ask them to remove your firearm's serial number from the NFA registry and nullify your stamp. Think of the stamp as the right to put your pistol into a SBR configuration at any time, and you can freely move back and fourth between pistol and SBR.

Does that help? Personally I say leave your SB01 on there if that is what you like and don't worry about it. If you want to temporarily put a real rifle stock on it from time to time, then apply and get the stamp.

Anthony L has some very good points. Not trying to take away from them, you have to actually understand the law that governs short barreled rifles. You cannot take a configuration such as an AR pistol and shoulder it and then it automatically becomes a short-barreled rifle. A short barrel rifle legally has to be a registered NFA weapon. Therefore it would have to be registered with the ATF and if it was owned by a citizen it would have to have a tax stamp associated with it. In summary, by shouldering a pistol that has an arm brace attached to it the person doing that is not using the arm brace for what it was intended to do, and the intention of the ATF. That is why the ATF will have difficulty Prosecuting anybody by shouldering an arm brace. The ATF has designated the arm brace just that an arm brace, not a stock. Such as a stock that you would find on an SBR. This is where the grey area is created. Now I am not a lawyer, and I'm not pretending to be, and I might not be the smartest person to interpret the law. But a violation of an intention or a letter or memorandum published by any federal agency falls into that gray area of what the actual law is. Such as the law that suggests constructive intent. If you have a standard AR-15, then you purchase a AR pistol upper. Could you be charged with constructive intent to use the AR pistol upper as an SBR by attaching it to your regular AR rifle lower? That would be difficult to prove, because of the AR-15 modularity inherent to the design, unless you were actually caught attaching the AR pistol upper to the AR-15 standard lower with the stock, then you could be charged with having a non-registered NFA weapon. I know these two examples don't directly correlate with each other it is an example of understanding the actual intention of the law. The ATF understands the cat is out of the bag. Once the ATF identified that arm braces are not an actual stock, they could not really go back on what they actually said. What the ATF did was they modified their initial statement by saying the intention of the arm brace is to use it as such, and arm brace not a stock. Personally, I would use your pistol as that. I would leave the arm brace on it, and use it as it's intended. If you are faced with a situation where as you had to use the arm brace , to secure it against your body in some form or fashion in a serious or potentially life-threatening situation, then it would be up to the legal process to actually prove that you were not using it as it was initially intended. That is the point at which the ATF is having difficulty trying to prove. I will say though with strict emphasis I would not want to be personally in a situation or a courtroom trying to prove that against the ATF. But, the ATF has made it quite clear that any arm brace is just that an arm brace. Not a stock. I use the Shockwave brace on my AR pistols, as well as the standard AR pistol buffer tube.

First, thanks for ALL the thoughtful replies!

While I'm digesting them fully, a slight tangent occurred to me: My pistol includes a buffer tube with a thin foam cover over the tube. If I returned it to that state (i.e., took the SB01 off and put the foam cover back on), is there ANY way the ATF could construe shouldering the pistol as it came from the factory to be temporarily "reconfiguring" the pistol into a short barrel rifle?

Per ATF Enforcement officers during an inspection (un-related to this subject), that letter is not binding. Not on official letter head or dated. Just a bunch of words on a piece of paper.

I honestly think you are over thinking this whole thing....

ME?

Photo taken after applying logic to ATF rulings.

Exactly. Trying to apply logic to state or federal firearm laws will only result in your own mental health deteriorating.

The answer is no. The foam covered buffer tube is not considered a 'stock'. That is actually the original configuration that AR Pistols came out it before the marketing of the 'Arm Braces'. So you can switch back and forth, you just cannot put a 'stock' on the AR Pistol , an item that is designated a 'stock' to which it can be 'shouldered' on a non registered NFA item; I.E- Short barreled Rifle.

Ok, but the SB01 is also not a rifle stock, and the process of shouldering one results in my "reconfiguring" my pistol into a SBR. If that is true then couldn't the same thing be said for a foam covered pistol tube, if I shoulder that tube don't I also create a SBR same as a SB01? ;)

I'm just poking fun, and highlighting these laws are ridiculous and I spend very little time worrying over them. Worry about the big picture stuff, are you building a pistol or SBR and follow the general widely accepted guidelines for each. Don't get caught in the fine detail interpretations...it will only make your head hurt.

If the ATF is to be consistent (and that's a big "if"), any pistol fired from the shoulder regardless of buffer tube hardware would be "redesigned" into a SBR.

If you want to fire it from the shoulder but don't want to cough up $200 and submit to federal registration, then keep the Sig Brace on it and ignore ATF's open letter. I seriously doubt anything will ever come of that BS.

If you want to fire it from the shoulder and have absolutely no doubt that you're in the clear, just make it a Form 1 SBR, put a real stock on it, and be done with it. It'll probably be slimmer and lighter that way...those Sig braces are thick and heavy.

That's exactly what I was thinking. They just insist on only disapproving one example at a time, and the principle they use can't be challenged because it's not written as law.

Conforming to ATF's revelations as to what gun laws mean is apparently as futile as halving a number again and again in the hope of getting to zero. It just isn't possible, because the next interpretation (in the case of the ATF) or division (in the case of numbers) is never the final one.

And you can quote me on that!

All points taken and considered.
Case in point, if you take a M92 PAP AK pistol, or any other AK pistol for instance or a CZ Scorpion, these do not have a buffer tube. If you put the end of the receiver against your shoulder and fire it, does it make it an SBR?
No.

Placing the weapon against your shoulder with these examples is not practical, but you could do it. The buffer tube on an AR pistol is just that, a buffer tube.