I shoot a lot of Aguila Colibri from an M6 Scout. It is about as loud and effective as an air rifle. The Super Colibri is a little louder and has a bit more velocity. Their sniper subsonic rounds do not "crack" like a rifle, but are not silent; but they carry a heckuva lot of energy down range.
Never had much luck with the CCI quiets, i was using a buddy's gun, but they seemed way to loud to be deemed "quiet". I usually stick with plain ole CCI long rifles, seem to work the best in a variety of rifles, including the lovable little cricket.
These students in this dorm I lived in used the loud FLUSH of a toilet to cover the sound of .22 short HP's fired from the bathroom at non-living targets of opportunity, not saying I was there or knew them or was flushing the toilet or anything.