Franklin Armory Antithesis, SBR without NFA regulations

It's not available here either, but what I think I'm seeing is the ammo itself is what won the case. Would that mean any AR-15 with a long enough free bore be permitted?

I don't believe the free bore has anything to do with it and this will likely branch out to include any rifle with just a bit of a push in the courts.

Way back in the 50's the Army experimented with multi-projectile 7.62x51 ammo in the 1950's so in theory any rifle chambered in 7.62x51 is by definition not firing only a single projectile.

While Winchester introduced the .308 Winchester in 1952 and the Army introduced the 7.62x51 in 1954, the reality is the cartridge existed as early as 1946 and was pushed along by powder developments that took place during WW2. They found they could achieve the same ballistics as the .30-06 with smaller case .30 caliber cartridges due to the new powder formulations, not the other way around.

So, multiple projectile rifle cartridges are not new. There was experimentation with the .45/70 government model back in the 1870's which used multiple round balls instead of a single projectile, so it is really an old idea. And any caliber can be adapted to it. I hope the NFA falls soon.