As has been said MANY times.. Little of the NFA/ ATF “rlues” many any sense.
It would make a heck of a lot of sense to reclassify a rifle vs pistol based on caliber. rather than OAL/ stock no stock. and eliminate a lot of the stupidity.
Classic examples of plain stupid are a Colt SAA/ Luger/ 1911/ broom handle with a but stock is a short bbl rifle..
And as had been said MANY times, the original National Firearms Act was going to subject all pistols with the same $200 tax as machine guns and silencers, so the SBR and SBS rules were added to close a loophole of using a sawed-off rifle in place of a pistol.
The pistol provision was removed from the NFA prior to passage, but the SBR and SBS provisions remained, giving us the ridiculous situation we find today, where an AR pistol chambered in 5.56 is legal, but putting a stock on it makes it an NFA item.