Per the ATF
The serial number must be engraved or stamped on the receiver of the firearm and the caliber, model,
and identification of the maker must be engraved on the barrel or frame or receiver of the weapon.96 The
marking and identification requirements for a maker are the same as for a manufacturer. Refer to
section 7.4 for a detailed discussion of the requirements.
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:
118 27 CFR 479.91
119 27 CFR 479.84
120 27 CFR 479.102
121 ATF Ruling 2002-6
(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.122
7.4.4 Obtaining variances to the marking requirements. Requests for variances from the marking
requirements of 27 CFR 478.92 and 27 CFR 479.102 should be submitted by letter to ATF’s Firearms
Technology Branch (FTB). The letter can be sent via mail to Chief, Firearms Technology Branch, 244
Needy Road, Martinsburg, WV 25405. The letter can also be sent to the marking variance e-mail
address at:
[email protected]. The marking variance request may be submitted by any of the
parties involved in the variance. However, if the primary manufacturer is in possession of all the
information including the names of the identity of the secondary manufacturers and the manufacturing
processes they may be performing on the firearm, it is preferred that the primary manufacturer submit
the request to FTB.
The marking variance letter of request should clearly state the following information:
• manufacturer, importer, or maker of the firearm(s),
• recipient of the firearm(s),
• identify the name, city and State that will be displayed on the firearm(s),
• model designation, if designated,
• identify the type/style of firearm (pistol, machinegun, short-barreled rifle, etc.),
•
caliber or gauge if assigned, and
• serial number scheme.
My understanding is that the caliber on your form 1 (they won't approve "multi"), must be marked in a conspicuous location. You are allowed to change caliber after the fact, but if you sell your original barrel and no longer possess one in the registered length and/or caliber, you have made a permanent change as you can no longer return the firearm to registered configuration.