Regarding the review , I have used an Attichison kit since the mid-70’s. (Yes, my first AR was bought over the counter in ’71, serial number under 20,000.) I have been satisfied with performance and accuracy. The CMMG is nearly identical so far I can tell on cursory examination, and I sold probably hundreds in retail and can’t recall any being returned or complained about. They save money, provide opportunity to train in places where .223 might prove problematic, and overall perform as they ought. I must confess to taking out a huge Norwegian Wharf Rat in my home in San Francisco’s Castro Market district back about ’77 using what was then called CCI Zimmerpatronen aka CB cap in a .22 LR casing. (This way it would manually feed through a .22 LR magazine), however with only a primed rim casing powering it and with the AR gas system absorbing some of the gas, the rat was dispatched and removed before the wife folding towels only 10 feet away entered the room, never having heard a thing. Someone in a restrictive urban environment could enjoy regular target practice in a residence basement using only a phone book for a backstop should they get the powderless CB .22’s in a long rifle case, of which several manufacturers are now producing.
The singularly most important consideration in using the conversion kit as such, a conversion kit, and after returning to using the .223/556 is that aside from residue accumulating in the gas system, usually blown out with the first round of .223, is that there is a slight residual dusting of lead and carbon that leaks around the chamber insert that builds up in the chamber to the point that if converted back to .223 without an absolutely thorough cleaning with a wire chamber brush and inspecting to assure all is removed, you risk destruction of the upper receiver. A friend here in Idaho fired zillions of .22LR in his M16 and then converted it back to .223; round #1 destroyed the upper but thankfully not the lower.
Enjoy using these kits, but I would prefer using a dedicated .22 LR upper, but the kit works great, so long as A CHAMBER FLAG is employed as mentioned in the article, AND a strict cleaning regimen is employed when converting back and forth from .223 to .22 and vice versa. – AHT