I did read through the other thread, just wanted to make that clear. My situation is this: I want to build another AR but I already have one in 5.56, am interested in another caliber. Had thought about 7.62 x 39 because ammo is cheap, but all the stuff I read about problematic magazines made me shy away from that - I don't want to spend several hundred bucks on something that doesn't work. So now I'm thinking 9mm since I have a couple of Glocks (17 and 23 w/9mm conversion barrel.) So my questions are on the 9mm AR platform itself. First of all, does the 9mm AR use the direct gas impingement of the 5.56 version or is it straight blowback? Reason I ask is that I know other 9mm carbines are straight blowback with an unlocked bolt. Also on the other 9mm thread I was looking at pictures of the bolt and it doesn't look like that bolt rotates, but maybe I'm not looking at it right? Next question, other than a 9mm upper, what else is needed? I know there's a magazine block to take the smaller mag but is there a separate magazine release or does it use the same one as the 5.56 version? Final question, are there any "CO Legal" < 15 rd magazines that will work in these? Thanks in advance.