ManxumFoe,
We are still in a state of flux on the SBRs. (not Flux Raiders

)
In the ShootingNewsWeekly article Para Pete linked I see a hole big enough to drive a truck thru - but I'm not a lawyer, or judge, or Franklin Armory.
Franklin Armory submitted the .45LC & .410 Antithesis. And the court ruled because it is designed to fire both rifle and shot shell it is not an NFA item.
From the ATFE letter linked in the article: "See 26 U.S.C. § 5845(c) (“
The term ‘rifle’ means a weapon designed or redesigned, made or remade, and intended to be fired from the shoulder and designed or redesigned and made or remade to use the energy of the explosive in a fixed cartridge to
fire only a single projectile through a rifled bore for each single pull of the trigger . . . .”); see also 18 U.S.C. § 921(a)(7) (similar)." -
emphasis added
So ATFE thinks - okay court ruled we can't regulate the 45/410 Antithesis.
But Franklin Armory saw the court ruling as - no NFA on things designed to fire both traditional rifle rounds and shot shell. So Franklin designed a 5.56mm shot shell. With that, pretty much any new short AR-type would be non-NFA.
Of course ATFE does not like that and tells Franklin Armory to stop.
I think the wording of 26 USC above indicates if a 5.56mm AR-type is labeled (or re-labeled) something like "Chambered in 5.56mm & 5.56 shot shell" (indicating it was designed or re-designed to fire both) then ATFE would not have an NFA case. So the writers of USC 26 did not foresee guns shooting both single and shot shell projectiles. Not a happy day for ATFE because it looks to me like they are still over stepping.
Further, I would point out any rifled barrel shotgun can fire both smooth slugs and shot shell so maybe those now dance around NFA/AFTE.