I'll be the first in a long line of ARFCOMers that will let you know that Cav is supposed to be making a forged aluminum lower that will allow you to use any AR15 stock you wish. We are all anxiously awaiting it's arrival.
BTW I put an .45acp Oly upper on my DPMS lower, and use the "funky" Oly blocked mags. They work great. I found high cap uzi mags at a gun show cheap and I'm having an ARFCOMer modify them for me, and will enjoy playing with those until the Cav aluminum lower comes in.
But hell, go with the wimpy 9mm if you want to.