Re: 10/22 Barrel Length
Aguila Colibri ammo comes out of my 27 inch barrel at 458 fps (average). It is completely silent, except for the "click" and the sound of the bullet hitting the target.
The same ammo fired through a 21 inch barrel has a bit of an actual report, and averages 512 fps, and through a 4 inch revolver averages 552 fps. The sound of this very light ammo through a 4 inch revolver is similar to a subsonic round through a rifle.
By the way, this particular ammo uses primer only, and is not particularly accurate, but it works 100% for mice inside 20 yards. Any farther, and I don't know about the accuracy or effectiveness.