Since I asked Chris to substitute my extra 9-mm hammer for the regular AR hammer in the parts kit, I could conceivably have three 9-mm carbines someday. One will definitely be a Short-Barreled Rifle, but my first priority is to get ONE working reliably. The culprit seems to have been the ejector on my Colt mag block being bent a little. A function test tomorrow will tell me if a little tug with pliers did the trick.
Update: Ordered a butt stock, buffer, another Colt mag block, and another 9-mm upper so I can have a complete, nearly identical back-up gun for the Arizona Sub-Gun Match next weekend. Yes, I know I could have bought a couple or three Hi-Point carbines for all that would cost, but I know and appreciate the AR's ergonomics, feel, and looks.