Re: CMMG 9mm AR lower
The Mk9 lowers/complete rifles/carbines generally run very well. I have had a chance to put several hundred rounds through two different models, generally without a hiccup. Now, with that said, both were finicky about mags. The Colt OEM/Metalforms ran like a scalded dog, but several promags and a few converted uzi mags (RRA branded) had FTFeed issues, especially when the mags were topped off (which I attributed to either weak springs, poor fitment of the mags in the CMMG magwell, or a combination of both).
Personally...I'm not crazy about the dedicated 9mm lowers (i.e. - those with the forged/integral 9mm mag well). While they generally work just fine, they don't offer nearly the flexibility of a multi-cal/standard AR lower that can obviously be used with all AR-15 compatible calibers, including the 9mm with an appropriate magazine block (I prefer the Hahn and Spike's blocks personally based on my experiences with various makes/models).