Throw out what you know while you read this.
I have a CAV-15 lower. I did not change the buffer, spring nor hammer to use my ASA 9mm upper. I just take the 223 upper off, put the block in the mag well and slap the 9mm on. There is a compatable hammer in 223 that will work w/ the 9mm upper. It is a hammer w/ a rounded top edge. NOT notched!!!
[img]http://photos.ar15.com/WS_Content/ImageGallery/IG_LoadImage.asp?iImageUnq=2002[/img]
The hammer on the left will NOT work. The one one the right WILL work.
The RRA 9mm and ASA 9mm are all Colt style/type 9mm systems. Original mag well blocks were two piece and then the single piece came out.
The two piece block from Brownells is a block that allows to use Colt mags/notched uzi mags. It must be MODIFIED to work w/ notched Sten mags. This was done by a few board members to creat their own block.
Now, ProfGAB101 as stated that he will be making some blocks to use nothed Sten mags in the Colt style 9mm. He as posted this online. You may want to contact him for further info or do a search for ProfGAB101. He posted it on page 8 and 9 of the thread that is tacked in this section "Sten Mags in Colt 9mm system"
Edit-
I was one of the original people who modified a JRW block (what Brownell's sells) to work w/ Stens. The others were Glftoe, AR-fan, HighlandMac and a bunch of others that I can't recall (sorry guys). This really works!!!
I have a Word file that has pics that describes the conversion. If you would like it for reference, email me and I will send it. Please note, that the file is about 1100K.