First new upper is a CMMG 4.5" .22 barrel in an AP slick side upper, using a Ceiner bolt w/ CMMG collar. My SBR Mega lower. Running Blackdog mags. Swapping the rail between the two. None of these subsonic .22 cycle the action. Quiet, though. Remington Gold Bullets cycle fine.
Second one is an ADCO 9mm 4.5" barrel, ADCO ramped bolt, in an AP slick side upper. My SBR Mega lower with Hahn drop in block. Running a Metalform and a couple ASCs. Federal Subsonic 124gr, hearing safe. American Eagle 147gr, hearing safe. WWB, hearing safe? Runs fine, though smokey.