OK, I finally have started buying the parts for my build.  This is what I have on the way:

1.  RRA 9mm A4 upper
2.  RRA 9mm bolt carrier set
3.  TROS 9.25" 3 lugged barrel (4th pic down)
4.  HAHN bottom loading 9mm block
5.  four 9mm Colt pattern polymber mags (I think they are ProMags).
6.  Troy Industries MRF-C
7.  RRA barrel nut
8.  MGI buffer

So the above leaves me with a couple of questions.

1.  Do I need a 9mm hammer?
2.  I know I need a barrel nut, but do I need the barrel indexing pin and barrel extension?  Looks like that is part of the TROS barrel.  I got those part names from RRA website here (parts 15 and 16).

Thanks for your help.  Without this forum, I would have given up long ago.

Quoted: OK, I finally have started buying the parts for my build.  This is what I have on the way: 1.  RRA 9mm A4 upper 2.  RRA 9mm bolt carrier set 3.  TROS 9.25" 3 lugged barrel (4th pic down) 4.  HAHN bottom loading 9mm block 5.  four 9mm Colt pattern polymber mags (I think they are ProMags). 6.  Troy Industries MRF-C 7.  RRA barrel nut 8.  MGI buffer  --- > why, a lot of extra money for something not really needed,  use the 5.5oz "9mm" for semi-auto if you using a CAR length tube...   So the above leaves me with a couple of questions. 1.  Do I need a 9mm hammer? NO, just the hammer in the middle with the spring still attached, and have the bolt ramped.   2.   but do I need the barrel indexing pin and barrel extension? NO Looks like that is part of the TROS barrel.  I got those part names from RRA website here (parts 15 and 16). #15 & #16 are for the 223/556 gas operated barrel...  plug the TROS barrel into the upper, secure it with barrel nut and run with it. Thanks for your help.  Without this forum, I would have given up long ago.

When you get your RRA bolt, knock the piece out that retain the extractor, and replace it with an everyday split type roll pin.   RRA seems to using some piece of scrap metal to retain the extractor, and everbody been having problems with rifle... scroll down the page here and see the comon RRA bolt problems appear in the topics,  2 or 3 currently here on the first page of this forum.

SBR7_11 - thanks for the reply.  I have been following the RRA roll pin issue and I am prepared to fix that already.  One more question if you don't mind.  Is the hammer with the spring a special hammer, if so, where can I get it?  Also, will it run both .223 and 9mm?

Thanks.

Quoted:  Is the hammer with the spring a special hammer, if so, where can I get it? It is simply a M16 hammer with the offending "lug" removed, bought it at the gunshow maybe 10 yr ago as seen in the pic..  DPMS sells this hammer for semi-auto use. Some other vendors are selling this hammer, but have to look around.   Also, will it run both .223 and 9mm? Yes.  This is the only hammer you will ever need, I just swap upper, magwell filler and go..  The 9mm hammer is just a COLT abortion. The lower-- 1 lower,  swap the tops... Thanks.