I can get a preban lower and preban (threaded) barrel for a GREAT price.  I have a post ban upper with no barrel in my spare parts box.  What is the feasibility of conjoining the parts?  I know the upper will attach to the lower, but what about the barrel to the upper?  I have done barrel swap outs before but never with a threaded barrel.  Are they compatible, do I need any special tools?  Also, I am assuming that since the lower with the Serial number is preban that all preban attachments are legal, even if the upper is postban?

I hate to bust on you, but are you effing serious?  In what alternative reality could the presence or lack of threads on one end of the barrel have any effect on the interface to the upper receiver at the other end?  

[thinking]

Like I said, I am unfamiliar with threaded barrels.  I assumed it was an older standard for attaching the barrel, preceeding the currently employed method.  This would be my first preban, and I was unaware if there was a difference.

So, you are saying that the Threaded side of the barrel is the business end?

AHA!  Now I see where the disconnect occurred.  Yes, when somebody referes to a pre-ban threaded barrel, they mean it is threaded on the business end to accept a flash hider.  The "threaded muzzle to accept a flash hider" is a banned feature.

All AR15s and M16s use the same barrel-to-upper interface and all are physically if not legally interchangable.

Unless the barrel is sans the barrel extension.  Then it is a threaded barrel at both ends, a more complicated install, for sure.

Everyone DUCK!!! [:O)]