From the NFA Handbook
https://www.atf.gov/content/firearm...tions-firearms-national-firearms-act-handbook
See 7.4.2
7.4.1 Serial numbers. Each manufacturer of a firearm must legibly identify it by engraving, stamping
(impressing), or otherwise conspicuously placing on the firearm’s frame or receiver an individual serial
number not duplicating any serial number placed by the manufacturer on any other firearm.120 The
requirement that the marking be “conspicuously” placed on the firearm means that the marking must be
wholly unobstructed from plain view. For firearms manufactured on or after January 30, 2002, the serial
number must be to a minimum depth of .003 inch and in a print size no smaller than 1/16 inch.
7.4.1.1 What is an acceptable serial number? Alpha characters (letters), for example a name,
are not acceptable as a serial number. A proper serial number may contain such characters or
letters, but it must have at least one numeric character (number). ATF takes the view that
marking “legibly” means using exclusively Roman letters (A, B, C, and so forth) and Arabic
numerals (1, 2, 3, and so forth).121 Deviations from this requirement have been found to
seriously impair ATF’s ability to trace firearms involved in crime.
7.4.2 Additional information. Certain additional information must also be conspicuously placed on
the frame, receiver, or barrel of the firearm by engraving, casting, stamping (impressing), that is, they
must be placed in such a manner that they are wholly unobstructed from plain view. For firearms
manufactured on or after January 30, 2002, this information must be to a minimum depth of .003 inch.
The additional information includes:1) The model, if such designation has been made;
(2) The caliber or gauge;
(3) The manufacturer’s name (or recognized abbreviation); and
(4) The city and State (or recognized abbreviation) where the manufacturer maintains its place
of business.
