With so much chatter on the Internet about frames and receivers, we thought a review of ATF marking requirements might be in order.  First off, serial numbers and other required information must be conspicuously engraved, cast, or stamped (impressed) on a firearm frame or receiver.  This is true even as a stand-alone part.  It is also true for finished firearms being imported into the U.S.  And it is certainly true for all complete firearms manufactured in the states.  Specifications?  A minimum depth of .003 inch and a point size of no smaller than 1/16 inch.  The serial number must be unique for each firearm, frame, or receiver.  And the placement must be such that it will not be susceptible of being “readily obliterated, altered, or removed.”  For a firearm, frame, or receiver imported after January 30, 2002, the following additional information must be affixed to the frame, receiver, barrel, or slide in like manner and to .003 inch:  (1) name of manufacturer, (2) country of origin, (3) model designation, and (4) caliber or gauge.  For an imported firearm, the markings must also include the country of manufacture, and the city and state of the import business.  For a domestically manufactured firearm, markings shall also include the name of the manufacturer, along with the city and state of the location of the business.  The ATF may authorize other means of firearms identification if an importer can demonstrate that the alternative is reasonable and will not hinder the effective administration of the regulations. For more information, turn to the ATF Guidebook “Importation & Verification of Firearms, Ammunition, and Implements of War – Firearms Verification.”  There are two statute sections, paired with two sets of regulations, that you may also wish to review at 18 U.S.C. §923(i) and 27 CFR §478.92, as well as 26 U.S.C. §5842 and 27 CFR §479.102.