It really doesn't matter if the sound reducing device is built in or attached, nor does it matter whether it is called a silencer, moderator, muffler, sound dampener, or whatever - or even if it was built so that a single discharge on a firearm will destroy it.. An airgun with a silencer almost surely is another arrest or lawsuit waiting to happen! At best, the owner of an airgun silencer may come out of court with only a few thousand dollars in legal costs and a felony record for a suspended sentence! The fact that many have been sold on the open market will not excuse the violation nor reduce the fact that the silencer user has given airgunning a bad name and a further push toward onerous restrictions!
n fact, U.S. Code specifically prohibits any federal, state or local municipality from declaring an airgun to be a firearm. Not only do the federal firearms laws NOT apply to airguns, they CANNOT be applied by law! But that doesn't get us out of the woods.
IF you have an airgun silencer, and IF it can be removed and installed on a firearm, and IF it then quiets that firearm - that silencer meets the legal definition and is subject to the law. However, in the United States, a person is considered innocent until proven guilty, so it is also necessary to prove intent, as in you intended to use the silencer in your possession on a firearm. Therein lies the problem.