I have shot some in my .22 Ruger GP100. They are a lot quieter than regular ammo but out of a short handgun barrel they are much louder than an air rifle. Out of my CZ 452 rifle with a 22.5 inch barrel they are about the same noise level as an air rifle.

It is harder to notice out of a handgun but CB caps are usually the most inaccurate .22 ammo you can buy in most guns. I have heard CCI quiet is just as quiet and much more accurate but have not been able to find any to try. CCI quiet uses a 40 grain bullet instead of 29 and is the same length as a regular 22 but like a CB cap is rated at 700 fps. If you can find that I buy it over CB caps.