After using a CMMG conversion kit with mixed accuracy results (and really dirty lower and upper), I built a dedicated CMMG .22LR upper with their BCG and the 1:16 twist .22LR 16" barrel.

This is a spendier option than just zipping out and buying an M&P-15/22 but it goes on a standard AR-15 lower, using all standard AR15 furniture and accessories (except for BBL, BCG, magazine obviously).

Mine went on a Bushy lower with RRA NM 2-stage trigger. I put the upper together with a $40 Anderson non-FA upper receiver, cheapie $60 free-float MLOK tube, no-name charging handle, and the barrel/BCG combo from CMMG. Little thing runs like a top and shoots great.