Quoted:
I don't have the law in front of me but I know the ATF passed a new law saying once a rifle always a rifle. If you make your stripped "other" lower you bought into a pistol you can switch the upper with a 16"er and slap on a butt stock. You legally have a rifle you can switch back and forth. But if you first build it into a rifle you can't make it into a pistol technically. How they would know which came first the chicken or the egg I don't know I guess you would have to take pictures to document it. For your own protection.
Btw multi just means the caliber denotation will let you run .223 5.56 or like someone else said "phased plasma"! (Good one)
Check your local laws and as always your mileage may vary.
Your right in a way. The ATF cannot pass any law. That would be like your local police passing a law. The ruling was changed to properly follow the existing law that was enacted by congress in 1968.
But yes, if it starts life as a rifle, it will remain a rifle unless you turn it into a NFA firearm which is a SBR or a firearm made from a rifle. That route is a form 1, $200 and a looooong wait.
A pistol/handgun may be converted to a rifle or other title1 firearm and then returned to a pistol/handgun provided it began life as a pistol/handgun.