18 USC 921(a)
(9) The term “importer” means any person engaged in the business of importing or bringing firearms or ammunition into the United States for purposes of sale or distribution; and the term “licensed importer” means any such person licensed under the provisions of this chapter.
(10) The term “manufacturer” means any person engaged in the business of manufacturing firearms or ammunition for purposes of sale or distribution; and the term “licensed manufacturer” means any such person licensed under the provisions of this chapter.
(11) The term “dealer” means (A) any person engaged in the business of selling firearms at wholesale or retail, (B) any person engaged in the business of repairing firearms or of making or fitting special barrels, stocks, or trigger mechanisms to firearms, or (C) any person who is a pawnbroker. The term “licensed dealer” means any dealer who is licensed under the provisions of this chapter.
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(21) The term “engaged in the business” means—
(A) as applied to a manufacturer of firearms, a person who devotes time, attention, and labor to manufacturing firearms as a regular course of trade or business with the principal objective of livelihood and profit through the sale or distribution of the firearms manufactured;
(B) as applied to a manufacturer of ammunition, a person who devotes time, attention, and labor to manufacturing ammunition as a regular course of trade or business with the principal objective of livelihood and profit through the sale or distribution of the ammunition manufactured;
(C) as applied to a dealer in firearms, as defined in section 921(a)(11)(A), a person who devotes time, attention, and labor to dealing in firearms as a regular course of trade or business with the principal objective of livelihood and profit through the repetitive purchase and resale of firearms, but such term shall not include a person who makes occasional sales, exchanges, or purchases of firearms for the enhancement of a personal collection or for a hobby, or who sells all or part of his personal collection of firearms;
(D) as applied to a dealer in firearms, as defined in section 921(a)(11)(B), a person who devotes time, attention, and labor to engaging in such activity as a regular course of trade or business with the principal objective of livelihood and profit, but such term shall not include a person who makes occasional repairs of firearms, or who occasionally fits special barrels, stocks, or trigger mechanisms to firearms;
(E) as applied to an importer of firearms, a person who devotes time, attention, and labor to importing firearms as a regular course of trade or business with the principal objective of livelihood and profit through the sale or distribution of the firearms imported; and
(F) as applied to an importer of ammunition, a person who devotes time, attention, and labor to importing ammunition as a regular course of trade or business with the principal objective of livelihood and profit through the sale or distribution of the ammunition imported.