Firing dry is not adding anything inside. Firing wet is adding a little (3-5 cc/teaspoons worth) water usually, but I have seen guys use rubbing alcohol, or even that gel hand sanitizer, to help cool the gasses inside which apparently helps make it a bit quieter. I know it works, but don't understand and can't explain the physics behind it. Personally, I shoot mine dry, and have always wondered about the wisdom of adding alcohol, which is flammable, to the inside of a suppressor, where it will be exposed to high heat that will act as an ignition source, but it seems to work, so may have to try it some day.