In before the Sig Brace Letter writers ;)
The internet is your friend. The ATF NFA web site has all the info you need regarding NFA items, do's, don'ts etc.
http://www.atf.gov/content/firearms/...l-firearms-act
http://www.atf.gov/firearms/faq/nati...-firearms.html
http://www.guntrustlawyer.com/2010/0...rements-f.html
The above mentioned information must be engraved, casted, stamped (impressing) or otherwise conspicuously placed or caused to be engraved, cast, stamped (impressed) or placed to a minimum depth of .003 inch and in a print size of the Serial number shall be no smaller than 1/16 inch.

