I have a M261 adaptor and it rests against the top of the buffer tube/receiver area. Mine has a sort of flat spring tensioner to keep the adaptor pressed up against the barrel. I'm pretty sure they all are secured that way and after looking at a cmmg unit it has the back plate with a nub that sticks up and abuts against the same area. After 15 years of use I noticed the spring had been squished and was no longer taking up the slack in the upper to keep it from hammering back and forth during recoil. I didn't notice any damage to my lower because of it and it still functioned normally. Sarco had a repair pack for it and I replaced the damaged spring.
Clean your chamber before switching to 223/556 or you can have a case stick. I found that out the hard way and it ripped off a case rim and bent the extractor pin. The 22lr in the ar15 seems quieter than in a 22lr rifle. The only thing I can figure is that some of the gas is getting diverted to the gas system and quieting the report.