From Rick V post #42:
A subsonic 22 AKA cb long is quieter than an air gun if used in a closed breach rifle. They will not cycle an automatic.
The crack you hear from a .22 is the round breaking the sound barrier, if you keep the round subsonic there is no crack.
I have removed several unwanted critters in my neighborhood with this setup.
https://forums.pelicanparts.com/off-topic-discussions/700757-groundhog-kill-semi-urban-hood-3.html