When I first bought y CMMG Mk9, I bought three mags from Brownell's. Talk about FTFs, OMG. None of the 3 worked consistently... well, they failed consistently. I told Brownell's and they took 'em back. (And I have bought a lotta products from Brownell's; great outfit, IMHO.) Called CMMG and asked for a recommendation. They pointed me to ASC, and I have never looked back. I took a quick look and I have about 15 of their Colt-style stick mags.
As a man once said, if it ain't broke, don't fix it. Good to know others have success with their choices, too!