If you have an AR pistol and an AR rifle, then you have a legitimate use for the stock and you would be ok. If you only had an AR pistol and a stock, then the only use for the stock is for the pistol and you would then have an SBR on your hands.
As for converting an AR pistol to a rifle and then back to a pistol, you cannot do that. Once it is a rifle it must stay a rifle. A lot of folks think that the Thompson Contender ruling allows this, but it does not. ATF lost the case, but determined that it only applied to TC kits that were sold as kits. All other weapons require ATF forms to convert a rifle to a pistol. I can post an ATF letter on this tomorrow when I get to my home computer (I am doing this on my phone now).
Oh, and do not try to apply common sense to firearms laws as there is none when it comes to them.
EDITED TO ADD: Here is the letter I was talking about.
Do you have a folder of ATF letters?
