Technically, that should read "starts a rifle, always a rifle." The ATF has repeatedly said that a pistol could be converted to a rifle, then back to a pistol again without penalty, but a rifle cannot be made into a pistol.
And it all goes back to the original 4473 that the firearm or stripped lower was first transferred from an FFL to the first owner. If the 4473 says "pistol" or "other," it can be a pistol or a rifle interchangeably. If it says "rifle," even if the item was a stripped lower, it can only be a rifle.
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..