18, United States Code, is amended— (1) in section 921(a), by striking paragraph (24) and inserting the following: (24) (A) The terms firearm silencer and firearm muffler mean any device for silencing, muffling, or diminishing the report of a portable firearm, including the keystone part of such a device. (B) The term keystone part means, with respect to a firearm silencer or firearm muffler, an externally visible part of a firearm silencer or firearm muffler, without which a device capable of silencing, muffling, or diminishing the report of a portable firearm cannot be assembled, but the term does not include any interchangeable parts designed to mount a firearm silencer or firearm muffler to a portable firearm.