Quoted: ... It uses a 9mm hammer, a Colt-style mag adaptor, and in my case (using with a post ban, A2 stock) required a 9mm heavy buffer, CAR spring, and a delrin buffer insert to account for the different lengths of the buffers.

Somebody said that a slight modification to the AR hammer would allow dual role with the RRA uppers which would then clear you to do the above.  Check here for more info -

www.ar15.com/forums/forum.html?b=3&f=15

If you go with an Olympic Arms pistol caliber upper you won't have to change buffers or make any hammer modifications.

RRA/Colt 9mm variants require the use of Colt 9mm mags or Uzi mags with a mag catch cutout.

The O.A. style upper has several options, the SOCOM mag block & the use of cheap Sten mags is popular.