I know that in order to have an 11.5" barrel, the rifle must be registered as a "pistol".  If I were interested in purchasing an 11.5" upper, what would I have to do to make it legal?  Would I have to re-register the lower?  Any help would be appreciated.

No you can just make it a SBR (Short Barreled Rifle) and your local Class 3 dealer will have all the paper work to help you out and even give some CLEOs who are willing to sign off on your stamp.

thanks dev

If the lower you use was stripped when it left the factory, you can make a pistol out of it and not run afoul of the NFA.  As far as I can tell, paying extra to get a lower registered as a pistol is BS.

The above point is very important because in the eyes of the ATF, if you start with a lower that had a stock at one time and place a sub-16" barrel on it, the result is an SBR [b]even if you remove the stock[/b].

I would just get a letter from the factory stating that the lower left stripped to be on the safe side.

I'm not planning to get an 11.5" barrel soon, I just wanted to know how to go about it if I do decide to get one in the future.  I think I'm going to get Bushmaster's new Carbon 15 pistol when it comes out.