If you are looking for a quiet 22 rimfire round, consider the Winchester SUPER-X CB-Match round. Winchester Ammunition introduced the .22 Long rimfire cartridge in 2008. The round is designed for high accuracy and very low noise.
The SUPER-X CB-match .22 Long round pushes a 29 grain bullet at a relatively slow 770 fps from a 24″ barrel. From a 6″ barrel, Winchester states the muzzle velocity is a paltry 575 fps. Expect even less velocity from a handgun like the Walther P22 (3.4″ barrel) or Ruger SR22 (3.5″ barrel.)
This round matched to a gun with a sound suppressor, and this should be a very quiet shooting round.
I would expect that this round would be reliable in only bolt-action and single-shot firearms. Running it in a semi-auto pistol or rifle may not work well, as the load probably doesn’t generate enough force to reliably operate the gun’s action. Of course, I could be wrong, and if anyone has any luck running this in a semi-auto gun, please sound off in the comments section below.
The ammo comes in 50-round boxes and 3000-round cases. I imagine a lot of people will buy this a the case level for fun plinking and varmint control. The 20-30 ft-lbs of energy this load generates should be plenty for rats and other such critters. I would also suggest this could make for a great first round for kids learning to shoot. The noise and recoil would be minimal and easy for kids to manage.
Winchester Ammunition is not the only company that makes a rimfire round designed for a reduced sound signature. CCI makes several .22 LR loads in the Quiet 22 line. These cartridges use a 40 grain bullet (round nose or segmented hollow point) traveling at 710 fps. I do not have any information on what barrel length the velocity was measured from.
The CCI Quiet 22 claims a 75% reduction in noise. Additionally, CCI claims shooters do not need hearing protection when using that ammo.