By the definition of a silencer in the NFA Handbook, absolutely, this oil filter adapter is considered a silencer. Big ol' fine and perhaps jail for using the part to quite your gun without proper approval.
From the NFA Handbook:
1.1.3 Firearm Owners’ Protection Act. In 1986, this Act amended the NFA definition of “silencer”
by adding combinations of parts for silencers and any part intended for use in the assembly or
fabrication of a silencer. The Act also amended the GCA to prohibit the transfer or possession of
machineguns. Exceptions were made for transfers of machineguns to, or possession of machineguns
by, government agencies, and those lawfully possessed before the effective date of the prohibition, May
19, 1986.
2.1.7 Silencer. A firearm silencer and a firearm muffler are defined as any device for silencing,
muffling, or diminishing the report of a portable firearm. Firearm silencers are generally composed of
an outer tube, internal baffles, a front end cap, and a rear end cap.
The definition of a silencer also includes any combination of parts, designed or redesigned, and intended
for use in assembling or fabricating a firearm silencer or firearm muffler.
The following illustration depicts parts that are designed and intended for use in assembling a firearm
silencer. Another example of parts redesigned and intended for use in assembling or fabricating a
firearm silencer are automotive engine freeze plugs that have been modified by drilling a hole through
their center to permit passage of a bullet.
Also included within the silencer definition is any part intended only for use in the assembly or
fabrication of a firearm silencer.
NOTE: the language in the definition of silencer contains no provisions that permit an owner of a
registered silencer to possess spare or replacement components for the silencer. However, licensed
manufacturers who are SOTs may possess spare silencer components in conjunction with their
manufacturing operations.
Any questions? Bad, bad, bad idea to use without approval.
