Permanently inoperable.

Permanently inoperable. A firearm which is incapable of discharging a shot by means of an explosive and incapable of being readily restored to a firing condition. An acceptable method of rendering most firearms permanently inoperable is to fusion weld the chamber closed and fusion weld the barrel solidly to the frame. Certain unusual firearms require other methods to render the firearm permanently inoperable. Contact ATF for instructions.

Source

27 CFR § 478.11

Scoping language

When used in this part and in forms prescribed under this part, where not otherwise distinctly expressed or manifestly incompatible with the intent thereof, terms shall have the meanings ascribed in this subpart. Words in the plural form shall include the singular, and vice versa, and words importing the masculine gender shall include the feminine. The terms includes and including do not exclude other things not enumerated which are in the same general class or are otherwise within the scope thereof.