if i bought a short barrel upper can i perm attach a long flash hider to make it legal length and it be legel?any one make long flash hiders for 9mm ar?  thanks!

I think the only ready-made FHs for 9mm are the A1 and A2 styles, which I believe are NOT long enought to bring you to 16".

If you are talking about 14.5",  you can take a Phantom and bore out the inside to allow a 9mm slug to pass through. Perm. attach it, and that will bring you to 16".

Just make sure your barrel is threaded 1/2 X 28 TPI.

If you are talking about something really short, I can't help ya.

Jake

http://www.armalite.com/sales/specials/newproducts/awbsunsetparts.htm

Try the .308 AR-10 version.  It is longer and is better suited for the 9mm.  It has a 5/8" thread, 24 pitch (I think).  

I believe the Phantom comes in the larger size too.  
You just need to find out who carries it.