Can you put a Rock River 9MM upper on a Rock River .223 lower? I hear that there are different hammers and buffer springs to consider when doing the swap. I would like to be able to swap back and forth. Any help would be great....

The lowers are the same, but you should use a 9mm hammer and buffer (I think the springs are the same).  Both parts are available from RRA.  I want to build one too as a SBR.

So once the 9MM hammer is installed will it work with the 223 upper??

You need 9mm hanmmer, buffer, mag block (w/mag)and complete 9mm upper (incl 9mm bolt assembly)

Right.. But once you have that set up and the 9MM hammer is installed will it work with the 223 upper. Basicly if I'm shooting my AR with the 9MM set up and want to switch to the 223 upper can i just change the upper and take out the magwell? I realize that the buffer spring may be different.    

Looking at the 9mm hammer in the latest RRA catalog I received the other day.  The 9mm hammer looks a lot like one of those ultra light hammers where they've just cut a lot off of the back side.  The 9mm uses a different buffer but the buffer springs are the same at least in the RRA 9mms.  So I think you could still use the lower with a .223/5.56mm upper by just changing out the buffer and removing the 9mm mag block.

This topic comes up every so often:

Most AR15 hammers look like this:
[img]http://www.biggerhammer.net/ar15/hammers/ar15_std.gif[/img]
however, the last ones that I got from RRA looked like this:
[img]http://www.biggerhammer.net/ar15/hammers/ar15_early.gif[/img]

If you have a hammer that looks like that latter, you don't need a special hammer.  The "stepped" face on the top illustration will hang up on the bolt GOOD, and will never function.  You can grind the step off though.  Alternately, you can ramp your 9mm bolt (joeblack does it, check this [url=http://www.ar15.com/forums/topic.html?b=3&f=15&t=163203&w=searchPop]thread[/url].)

As far as the buffer spring and buffer go, just run with what you have.  I have a RRA lower, with an A2 stock, .223 buffer and .223 buffer spring, and a hammer like the one pictured in the bottom picture (and my bolt's not ramped), and it runs absolutely 100%.  Any more question, you can feel free to email me!

-Gloftoe