18 U.S.C. § 931 : US Code - Section 931: Prohibition on purchase, ownership, or possession of body armor by violent felons

      (a) In General. - Except as provided in subsection (b), it shall
    be unlawful for a person to purchase, own, or possess body armor,
    if that person has been convicted of a felony that is - 
        (1) a crime of violence (as defined in section 16); or
        (2) an offense under State law that would constitute a crime of
      violence under paragraph (1) if it occurred within the special
      maritime and territorial jurisdiction of the United States.

      (b) Affirmative Defense. - 
        (1) In general. - It shall be an affirmative defense under this
      section that - 
          (A) the defendant obtained prior written certification from
        his or her employer that the defendant's purchase, use, or
        possession of body armor was necessary for the safe performance
        of lawful business activity; and
          (B) the use and possession by the defendant were limited to
        the course of such performance.

        (2) Employer. - In this subsection, the term "employer" means
      any other individual employed by the defendant's business that
      supervises defendant's activity. If that defendant has no
      supervisor, prior written certification is acceptable from any
      other employee of the business.