A person who is licensed under the Gun Control Act (GCA) to manufacture firearms and who has paid the special (occupational) tax to manufacture National Firearms Act (NFA) firearms may replace a component part or parts of a silencer as long as the repairs do not result in removal, obliteration, or alteration of the serial number. If a silencer part bearing the serial number, other than the outer tube, must be replaced, the new part must be marked with the same serial number as the replacement part.
The term "repair" does not include replacement of the outer tube of the silencer. The replacement of the outer tube is so significant an event that it amounts to the "making" of a new silencer. As such, the new silencer must be marked, registered and transferred in accordance with the NFA and GCA.
In the event that identical replacement parts for a silencer are not available, new and different component parts may be used as long as the silencer retains the same dimensions and caliber. In addition, the repair may result in a minimal reduction in the length of the outer tube due to re-threading, but repair may not increase the length of the outer tube. Increasing the length of the outer tube significantly affects the performance of the silencer and results in the "making" of a new silencer. Reducing the length of the tube by a minimal amount in order to repair a silencer is often necessary to replace damaged end caps, as the tube must be re-threaded. Such minimal reduction of the length of the tube uses all of the original parts, does not significantly affect performance of the silencer, and may be done as part of a repair process without making a new silencer.
Persons other than qualified manufacturers may repair silencers, but replacement parts are "silencers" as defined in 18 U.S.C. § 921(a)(24) that must be registered and transferred in accordance with the NFA and GCA.