Howard,

As long as you are not in possession of a lower receiver, or another complete AR, you are okay. But, as soon as you have enough pieces to assemble a complete AR, the 11.5" barrel is a no-no unless you have an SBR stamp for the lower (or a registered M16).  If you have the 5.5" flash hider permanently attached (to a total length of 16" or more) you can avoid the NFA regulations on short-barrelled rifles. If you plan on having the flash hider pinned and welded, just don't have the lower in the same location. You need to ensure that you are not in possession of both the short-barrelled upper and the lower together.

Quoted: The seller describes it as 11.5/5.5. Ive known the guy for years from another forum and trust(hope) he doesnt send anthing illegal.

That barrel configuration should get you at or over 16" without any problem.  Short of something goofy, you are fine.



You may want to try this post in the builders forum.  There is a lot of good info there, and the same questions have been asked and answered many times.  You may find answers to things you haven't even thought of yet.



ETA-  For used items, check the equipment exchange (EE), and welcome to the boards!