If I put a rifle upper on a pistol lower......can I shoulder the SB3 pistol brace legally ???

You can shoulder it with any upper.

FPNI

Yes

Thanks

You can make a rifle from a pistol and go back to a pistol configuration, because as a pistol configuration, the gun was subject to the more stringent ownership requirement

Not everywhere.  It takes no more to buy an AR pistol than a AR rifle here in MN, and most other states.  But, to buy an AR rifle we have to go through the same ordeals as a pistol, same as a stripped lower.  Some states less, some more.

My idea is to be able to quickly switch between a pistol and rifle by just changing the upper.

Yes, that’s fine as long as you start out as a pistol. And as long as you don’t have any accessories on it that changes it to a rifle or SBR when you change it back to a pistol. Such as a stock instead of a brace or a vertical foregrip.

That's not at all in any way, shape, manner or form the reason why a person can reconfigure pistol>rifle>pistol.
US vs Thompson Center is why.

And, there is not a "more stringent ownership requirement" in Federal law for pistols.
Federal law prohibits licensed dealers (I is one) from transferring any firearm to a person under the age of 21. It provides an exemption for rifles and shotguns to those buyers who are eighteen years old.

I may be wrong, but if you start as a pistol with a braced lower, swapping a 16” upper shouldn’t change the legal classification of the firearm.
You can have a pistol with any length barrel. If it goes over 26” it may be an AOW but that doesn’t change it to a rifle.

But for the OP, shouldn’t matter, you can legally shoulder a braced pistol, period. Now if you go to a range that doesn’t allow braced AR pistols they probably won’t give a 16” build a second look.

I think the big question is whether your lower was registered as Other or Pistol. If it’s Other ( purchased as a stripped or complete lower) then build it in any legal configuration you want. It’s still Other. But if it was purchased as a complete pistol, will adding a 16” upper legally make it a rifle? I doubt it.

I guess what I’m trying to say is if you’re worried about the legality of using or operating a brace, going to a 16” barrel isn’t gonna change anything. Unless that lower was registered as a rifle and just happens to wear a brace. But then any pistol upper would make a rifle lower an SBR.

Nope. Nope. Nope.
"Braced" pistols with an OAL of 26" or more that have a second vertical grip would be just a "firearm".......because such a firearm does not meet the definition of pistol/rifle/shotgun/receiver/frame as a Title I gun.. If less than 26" it would be an AOW (Any Other Weapon) with that second grip....requiring an NFA tax stamp.  



1. There is no "registered as" in Federal law or the majority of states.
2. An AR lower, a frame, receiver or firearm that expels a shotgun shell.........all transfer as an "Other firearm". There is no such animal as a "pistol lower" or "rifle lower"....just lower.
3. If the lower was first assembled into a rifle....its a rifle forever.
4. If the lower was first assembled as a pistol, it can subsequently be reassembled into a rifle and back to pistol as often as desired.

I was quite interested in buying the blackpowder Thompson Center Contender rifle, then i asked and found out it transfers as a pistol.

If it is blackpowder only, you don't even need to go through an FFL.  If it is  a switch-barrel frame, it can be used for pistol barrels so then yes

Not true.
"Blackpowder only" has nothing to do with anything.
Muzzleloaders using black powder are almost always exempt from GCA '68...some are not (Rem 700 and Traditions Nitrofire)

T/C Encore frame/receivers can also be used for centerfire or rimfire cartridges, so are not exempt from GCA '68.