Here's something I ran accross and tried for my Ruger 10-22, sub-sonic 22 ammo. They are made by CCI, 22 CB longs. I fired a few in my back yard and they sounded like a pellet gun. My kids in the house didn't hear me shoot out the back door. I was able to shoot through a 2x6 laying on its side and the bullet was about 2 inches deep in the sand. So they seem to have power even though they are slow and very quiet. I think I paid about $7.00 for a hundred from midwayusa.com
I forgot to mention that these low power 22 rounds will not actuate the bolt in a semi 22. In my Ruger I have to manually eject the shell. A bolt action would be fine.