Thought I'd report on my CMMG "Sierra" .22LR upper. First I have to say that I'd considered my options and was leaning to TacSol (which I'm sure is a great product); however, the price differential with the CMMG was significant (I got mine for $349 plus $20 for a Black Dog mag). I mated the upper with a Stag lower with no problem, installed a Nikon 3x9, and headed for the range. First time out was a little bit of a letdown as the polymer CH broke at my first attempt to load a round. So I put in a spare alum. CH and proceeded to break in the new toy. Later got a replacement polymer CH from CMMG CS with no problem, and it has worked fine since. The only other problem was that the unit didn't seem to like Win. SuperX Hi vel. (it would feed but not fire them). Fed. bulk, CCI Minimags and CCI Standard all worked fine and displayed very good accuracy. There have been a few stovepipes with the Fed. bulk. Eley Sport yielded great accuracy. After the first 100 rounds I tried the Win. again and it worked fine. The unit now has over 800 rounds thru it with no issues other than the few mentioned previously.
The Sierra upper comes with a blued BCG, and I would suggest an upgrade to the SS BCG for ease of cleaning. Otherwise, I am quite pleased and recommend this upper highly. WB