First , any legal problems building a 9MM if I keep everthing POST BAN except for using this caliper?
Second, Can I build it on a normal forged ( rock river ) lower and purchase a complete assembled 9mm upper?
Third, I understand I need the magizine adapter but what clip do it use?

No problem putting a 9mm upper on a regular AR15 lower.  The 9mm uppers as a general rule will fit and work on any AR15 lower.  Sometimes you need to do a little filing on the upper to get it to fit with certain lowers, depending on what upper and what lower you use.  If you are using a post-ban lower, then you have to make sure that the upper conforms to the assault weapon ban regs.  What kind of adapter and what mags you will need depends on which upper configuration you use.  If you go with the Oly Arms upper, you can use individually modified Sten mags or you can buy one of the SOCOM mag blocks and use Sten mags.  If you are going to use one of the uppers that is based on the Colt configuration, then you will be limited to using Colt factory mags or Uzi modified mags.  With the Colt configuration, there are several different manufacturers of adapters you can choose from.

Charles Tatum
Alamo Professional Arms

Well, there is a way of using notched Sten mags in the Colt style 9mm.  Several member (I included) modified a two peice block block to work with Sten (notched; so the 223 mag release will work).  This was a custom modification.

The user ProfGAB101 has stated that he is making some of these blocks and will be offering them up for sale.  Up until mentioned he was making some, no one made Sten blocks for the Colt style 9mm (unless it was custom made).

Also see [url=http://www.ar15.com/forums/forum.html?b=3&f=15] Rimfire and pistol Calibers[/url] there is a ton of info on the different 9mm uppers.

Jamie