Again, right on the BATFE website, there is documentation on this issue.
http://www.atf.gov/firearms/faq/faq2.htm#m30
If it can silence a paintball gun, airgun, or BP gun, IT'S A SILENCER.
http://www.atf.gov/firearms/faq/faq2.htm#m30
Numerous paintball and airgun silencers tested by ATF’s Firearms Technology Branch have been determined to be, by nature of their design and function, firearm silencers. Because silencers are NFA weapons, an individual wishing to manufacture or transfer such a silencer must receive prior approval from ATF and pay the required tax.
If it can silence a paintball gun, airgun, or BP gun, IT'S A SILENCER.