The only ATF Regulation I am aware of that even touches this is
THE GUN CONTROL ACT OF 1968
TITLE 18, UNITED STATE CODE, CHAPTER 44
§ 922 Unlawful acts.
(a) It shall be unlawful—
(1) for any person—
(A) except a licensed importer, licensed
manufacturer, or licensed
dealer, to engage in the business of
importing, manufacturing, or dealing
in firearms, or in the course of such
business to ship, transport, or receive
any firearm in interstate or foreign
commerce; or
(B) except a licensed importer or
licensed manufacturer, to engage in
the business of importing or manufacturing
ammunition, or in the
course of such business, to ship,
transport, or receive any ammunition
in interstate or foreign commerce;
(2) for any importer, manufacturer,
dealer, or collector licensed under the
provisions of this chapter to ship or
transport in interstate or foreign commerce
any firearm to any person other
than a licensed importer, licensed
manufacturer, licensed dealer, or licensed
collector, except that—
(A) this paragraph and subsection
(b)(3) shall not be held to preclude a
licensed importer, licensed manufacturer,
licensed dealer, or licensed collector
from returning a firearm or
replacement firearm of the same kind
and type to a person from whom it
was received; and this paragraph
shall not be held to preclude an individual
from mailing a firearm owned
in compliance with Federal, State,
and local law to a licensed importer,
licensed manufacturer, licensed
dealer, or licensed collector;