CCI makes a round called a "22 CB" it is a very low velocity, low gunpowder round. i can shoot it out of my Ruger 10/22 and it sounds just like a bb gun. it has about a 1/8 mile range and isnt powerfull enough to eject itself so the 10/22 acts like a single shot. its very quiet for plinking in the backyard and not disturbing the neighbors.

forget about using a tennis ball or soda bottle on the end of your barrel, that will just screw up your aim.