Stopped by my favorite Albany, NY gun shop yesterday to check out what they had.  I live in Rochester so I stop in whenever a service call, (or 4) puts me in the area.  They had a couple of complete Rock River Arms lowers.  So, out came the debit card and now I have a lower to go with my FN 20" HBAR  A2 upper.  The price was fair to me, so since I haven't had time to assemble anything at home lately, I figured what the heck.   I got home a few minutes ago and mated the two together.  A match made in heaven.   I may even get it to the range this weekend.  

What impressed me was the quality of the receiver.  I have never handled a RRA rifle before so it was a pleasant suprise to see the quality of the finish and machining.   Mags fit nicely, fall out as they should and the trigger pull is pretty good too.   Now I got another complete rifle to sit with my Bushmaster and Olympic.  

Does the barrel of you upper have FNMI stamped on it?

Yes, the barrel does have FNMI on it.  It also has a 1/7 twist.