While we're on the topic of the modern 'CB' loads, don't forget the CCI loading marketed as 'CB Long' which is the short conical bullet in .22LR brass.
It's subsonic, and in every rifle I've shot it in - very accurate.
In my 9mm/.22 cape gun and my Anchutz it's silent, quieter than my .177 air rifles.

I can't say for sure what the propellant is or what the chamber pressure would be in any particular gun.
Perhaps a note to CCI would be in order to answer that kind of question.


--Tinker