I would like to make one point here, though I state up-front that I'm a newb.
In any case, I did some reading, where I could, about the CMMG .22 LR conversion kits. What I believe to be true, from what I read, is that initially they were not stainless, and some had issues. The ones now are stainless/new models, and most of the "bad" reviews I saw were from the OLD style (if I am not mistaken). The reviews, from what I can tell, have been more positive on the new stainless ones.
I just purchased a new M&P Sport, and the CMMG "Bravo" stainless conversion kit. I haven't fired a round through the rifle yet (5.56, .223, OR .22LR), but will, soon. I can report back my results, once I have, but I wanted to share that thought (about the "old" conversion kits vs. the new ones), at least as I understand things (please someone feel free to correct me if I'm wrong).
I do realize that given the barrel twist, the Sport's barrel is not optimized for such a small round as a .22LR, and so I expect it will not be as accurate as a dedicated .22LR rifle. However, accuracy is not my overriding concern when shooting .22LR; becoming more proficient with my weapon, is. My plan is to shoot lots of .22 each trip to the range, and then finish up with a mag or two of .223/5.56. There may be flaws in my plan, or in my equipment (conversion kit), but I can adjust as I learn.
Steve