OK folks....if I have a Colt 9mm subgun upper, do I need a special 9mm auto hammer in my RR lower to run a Hahn block?  Or does the regular M16 hammer work?

The regular hammer will work.  The down side is, without ramping the bolt like a .223 carrier is, it will beat the daylights out of your firecontrol pins.  The standard 9mm bolt will pound the pins so hard they can break.  Worse yet, the pounding can egg shape the fire control holes in your lower.

I would highly suggest that you install a set of KNS anti rotational fire control pins and have the 9mm bolt ramped.  I got my KNS pins from Denny's Guns and the bolt ramped by M60Joe.  Both of them I contacted through the EE Board.  I would also suggest a MGI rate reducing buffer.  I use mine in my carbine stock.  It will handle 9mm, 5.56 NATO, and 7.62X39.  Good luck with your 9mm project.

Scott