I'll add my 2 cents here... I have been using standard .22 shorts, and they are NOT quiet at all. They have a definite "crack" when fired that will not be mistaken for a pellet gun.
Just yesterday, I started trying the CB shorts in my back yard. I can honestly say that these are very quiet. I shot a medium sized bird, think Blue Jay size, and the bullet did not exit. It dropped the bird from the tree about 35 yards out. The bird was not dead on impact with the ground however. It was kicking around on the ground. I had to take another CB short and pop it in the head to finish it off. After that, I was shooting into some wood that I had set up as a back stop. It will penetrate 2 pieces of 1/2" plywood, but stopped in the 3rd. So, I can say that if I need to kill something in my backyard, I will be using these.
There is NO comparison the two.