Sounds an awful lot like the Ciener .22 conversion kit, and the mags that are made by Blackdog Machine.
Yeah but with the cmmg you don't have to deal with an a_-hole if something goes wrong with your ciener conversion.
On the website CMMG says the conversion is for semi-auto only. Unless they cam out with one for select-fire lowers. A friend of mine has the cmmg conversion and it seems to be working well for him.