[Don H]

The BATF is of the opinion that suppressors on paintball guns and airguns are NFA weapons. I can't see how they'd think muzzleloaders were any different.

Are Paintball and/or Airgun Sound Suppressers NFA firearms? [Back]

The terms "firearm silencer" and "firearm muffler" mean any device for silencing, muffling, or diminishing the report of a portable firearm, including any combination of parts, designed or redesigned, and intended for use in assembling or fabricating a firearm silencer or firearm muffler, and any part intended only for use in such assembly or fabrication.

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.

http://www.atf.gov/firearms/faq/faq2.htm#m30