I appreciate this thread. While I’m not into ARs I had a wondering myself. Thanks.
As you can see this covers more than just AR pistols and AR rifles. The case; US vs Thompson Center is what caused the ATF to clarify things with the ATF Ruling 2011-4
https://en.wikipedia.org/wiki/United_States_v._Thompson-Center_Arms_Co.
The case US vs Thompson Center was due to their kits they sold. Thompson sold their Contender pistols that included a shoulder stock and a 16+ inch barrel in the box. The ATF tried saying it was too easy for people to make an unregistered SBR with the Contenders that came with a stock and rifle barrel. The courts decision was that as long as it was in a pistol configuration FIRST then it could be changed to a rifle and then back to a pistol. The main thing is that it stays in a legal form at all times.
This is true with any pistol that can be easily converted to a rifle and back. As long as it starts as a pistol first, then you can legally go to a rifle and then back to a pistol. The main thing to remember is to NEVER had the stock attached while a barrel less than 16" in OAL is attached.
When changing your pistol to a rifle, then you need to attach the rifle length barrel first before attaching a shoulder stock.
When converting back to a pistol, you need to remove the shoulder stock first before attaching the pistol length barrel.
As you can see it started with the Thompson Contender but also covers firearms such as AR's, Ruger Challenger, or any break action and bolt action pistols. Remember by federal law, it must start out as a pistol first.
Also make sure to check your state laws since they can be ore restrictive.
PS;
@dogtown tom I tried your link to the US vs Thompson case and got a message that the article could not be found so that is why I posted a different link .