Here is part of the code.
It shall be unlawful for any person to sell or otherwise dispose of any firearm or ammunition to any person knowing or having reasonable cause to believe that such person—
(1) is under indictment for, or has been convicted in any court of, a crime punishable by imprisonment for a term exceeding one year;
(2) is a fugitive from justice;
(3) is an unlawful user of or addicted to any controlled substance (as defined in section 102 of the Controlled Substances Act (21 U.S.C. 802));
(4) has been adjudicated as a mental defective or has been committed to any mental institution;
(5) who, being an alien—
(A) is illegally or unlawfully in the United States; or
(B) except as provided in subsection (y)(2), has been admitted to the United States under a nonimmigrant visa (as that term is defined in section 101(a)(26) of the Immigration and Nationality Act (8 U.S.C. 1101 (a)(26)));
(6) who [2] has been discharged from the Armed Forces under dishonorable conditions;
(7)
who, having been a citizen of the United States, has renounced his citizenship;
http://www.law.cornell.edu/uscode/text/18/922