The law on "engaged in the business" DOES have a "for profit" element.
Quote from ATF, quoting the actual U.S. Code:
"The term “manufacturer” is defined by 18 U.S.C. 921(a)(10) as any person engaged in the business of manufacturing firearms or ammunition for purposes of sale or distribution. As applied to a manufacturer of firearms,
the term “engaged in the business” is defined by 18 U.S.C. 921(a)(21)(A) and 27 CFR 478.11, as 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."
https://www.atf.gov/file/82791/download
Quote from ATF, quoting the actual U.S. Code:
"The term “manufacturer” is defined by 18 U.S.C. 921(a)(10) as any person engaged in the business of manufacturing firearms or ammunition for purposes of sale or distribution. As applied to a manufacturer of firearms,
the term “engaged in the business” is defined by 18 U.S.C. 921(a)(21)(A) and 27 CFR 478.11, as 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."
https://www.atf.gov/file/82791/download