It is 18 USC 922(a)(6):
http://www.law.cornell.edu/uscode/18/usc_sec_18_00000922----000-.html
(a) It shall be unlawful—
(6) for any person in connection with the acquisition or attempted acquisition of any firearm
or ammunition from a licensed importer, licensed manufacturer, licensed dealer, or licensed collector, knowingly to make any false or fictitious
oral or written statement or to furnish or exhibit any false, fictitious, or misrepresented identification, intended or likely to deceive such importer, manufacturer, dealer, or collector
with respect to any fact material to the lawfulness of the sale or other disposition of such firearm or ammunition under the provisions of this chapter;
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