sorry, didnt mean to ruffle any feathers. please ignore me. ill go back to lurking now
No worries! I think Dave is referring to the fact that the ATF considers installing one of these kits onto your handgun frame to be making your handgun into a rifle. (It IS a carbine at that point, and a carbine is a kind of rifle.)
If it IS a rifle, then according to the National Firearms Act of 1934, it is illegal to make a firearm out of it that is less than 26" long or has a barrel of less than 16". Because of the odd wording of the law, that DOES include making it back into a normal pistol.
So, the ATF views this as a "one-way trip." If you want to reinstall your handgun barrel after you've converted the pistol into a rifle, you'd have to first register the gun as an NFA, Title II, short-barreled rifle.
As logically absurd as the law is -- it IS written pretty clearly.
So, absent a willingness to commit a federal felony, or pay a $200 tax and file federal paperwork, you have to have a pistol frame you want to permanently devote to rifle duty.
(FWIW, many people say they'll never be caught or prosecuted. And, MechTech says they disagree with the ATF's opinion. But, they won't be the ones prosecuting or defending you if you end up in trouble. Their opinion and a dollar bill will get you a cup 'o joe at McDonalds, but that's about it.)