The 26" overall length rule has more to do with whether you can attach a vertical grip on it, which further determines whether it's a rifle or a pistol.

If you have a pistol that is UNDER 26" in overall length, you cannot legally attach a vertical grip, or it becomes a SBR and requires registering.

If you have a pistol that is OVER 26" in overall length you can legally attach a vertical grip, because according to the ATF, it's no longer concealable as a pistol.

The proper way to measure overall length of a pistol is to remove the brace, and measure from the end of the buffer tube to the muzzle...or muzzle break if it is permanently pinned and welded.

Outside of the vertical grip 26" OAL deal, pistol/rifle comes down to whether the barrel is 16" or not.

You can run a brace on either rifle or pistol...doesn't matter.

You can only *legally* run a stock on a rifle, or registered SBR.

A lower originally sold as a "pistol" or "other" can be converted to a rifle, and back to a pistol later...and back and forth freely...legally. Just as long as you don't pin a "pistol" upper (under 16 permanent inches) on a "rifle" (stocked) lower. It either has to be a pistol, or a rifle. It can never legally be a SBR unless it is registered.

It really makes little sense, but that's the law.

I hope that's clear.