Recently built a dedicated upper using a CMMG .22 9" barrel and bolt. It works great with M&P mags and I would highly recommend the Boonie Packer adapter. I considered the spacers that you had to glue to the mags but didn't want to jack them up. Plus having both the CMMG and the M&P 15-22 on the line at the same time is pretty fun. The other benefit of the Boonie Packer is last round hold open after you drop the mag. Most of the adapters I found didn't offer that

As for reliability, I think I have about 500 rounds of bulk pack through it since building it in September without cleaning. All but maybe 10 rounds has been suppressed and it's run great so far. I will say unsurprisingly that it gets extremely dirty though... Shocking I know...

The upper I built is also a good size and weight for younger shooters. Had my friends 10 year old working up drills and he never complained about the weight or getting tired.