The CMMG upper didn't used to actuate the bolt hold open. You had to get an add on kit and use special magazines.
I don't know if that is still the case or not.
Fishbed77 probably knows for sure.
Personally, I went with the M&P 22-15.
I have one set up as close as possible to my Colt 6920, I have the other setup as a lightweight tree rat rifle.
Significantly lighter than both of my other tree rat eradicators a Marlin 39a and a Marlin 881.
I am no longer participating in gun forums.