https://www.atf.gov/firearms/qa/are-paintball-and-airgun-sound-suppressers-considered-nfa-weapons
By nature of their design and function.
Then there is this old 2006 article from Pyramid Air:
https://www.pyramydair.com/article/Airgun_silencers_What_s_the_big_deal_August_2006/32
and an update:
https://www.pyramydair.com/blog/2019/03/silencers-an-update/
Here's a link to their air gun silencers:
https://www.pyramydair.com/accessories/misc/silencers Please note the descriptions carefully as well as they seem to be muzzlebreaks and candidly admit they dont reduce the sound.
Notably, there doesn't seem to be very many companies making actual silencers that attach to airguns. I did find some posts from a nitwit that claims because airguns arent firearms (true) a silencer for one cannot be regulated by federal law (false)........thats because silencers and parts of a silencer are considered firearms by themselves.
ATF Ruling 2005-4 is in regards to Paintball guns with ported barrels but needs reading
https://www.atf.gov/firearms/docs/ruling/2005-4-ported-device-attached-barrel-paintball-gun/download
",,,,,,,The sole issue presented is whether the ported barrel and outer sleeve are a
firearm muffler or firearm silencer as defined in the GCA and NFA.
The design characteristics of the ported barrel and outer sleeve are similar to
those of conventional commercial silencers. The barrel is ported to allow the
escape of gases from a fired round and the outer sleeve dampens or muffles the
sound when a round is fired. Moreover, the sound meter test indicates a reduction
of 7.98 decibels when the ported barrel and sleeve were attached to a .22 caliber
pistol, which is consistent with the sound reduction resulting from the use of
commercial silencers.
Noteworthy, the definition of “firearm silencer” and “firearm muffler” requires that
the device be one for diminishing the report of a portable firearm. The device
under consideration is permanently attached to and an integral part of a paintball
gun, which is not a firearm as defined in the GCA or NFA. The device cannot be
removed from the paintball gun without destroying the barrel and rendering the
paintball gun unusable. Under these circumstances, the integral device is not a
firearm muffler or firearm silencer.
However, once the device is cut from the paintball gun, it can be used to diminish
the report of a firearm. As stated previously, the design characteristics of the
device are consistent with those of commercial silencers, and testing indicates that
the device functions to reduce the report of the firearm. Moreover, removal of the
device from the paintball gun indicates some intention to utilize the device for
something other than reducing the report of the paintball gun. Because the device
will no longer be permanently attached to an unregulated item, and because of its
silencer design characteristics, removal will result in the making of a silencer under
the NFA and GCA. This is consistent with the definition of “make” in the NFA, as
removal of the device results in production of a silencer......"
It seems as long as the airgun incorporates an integral silencer it is fine (as the above ruling explains). But if removable, it isn't going to matter what threads you have on it....it needs a tax stamp.
