I read an article yesterday that talked about paying the $200 and getting a registered lower receiver.  I think it was about having a lower that one can legally put a short barrel upper on.

What does this ,mean?  Can I send in my serial number and register my lower so I can use short barrels on it?  Anyone have info on this topic?  I would like to have a short barrel AR to use as an entry weapon.

SBR FAQ
SBR section

If barrel is less than 16", and OAL is 26", you have a SBR. $200 tax, Form 1, 2-4 months, etc.

Most of the time when someone say's " registered receiver". they mean a M-16, or full  auto