The foam is what makes it actually "work" as a silencer (as much as that's possible with an AEG); without the foam, your silencer becomes an airsoft-actuated bell.
EDIT: Yes, that's normal; the silencer is containing expanding gasses, so-to-speak, much as a real one would do.
Delenda Est Carthago
Last edited by Frozen Tex; September 30th, 2009 at 17:54..
Reason: Answering a question.