The ATF does regulate the manufacture and sale of ammunition. However, making and selling a reasonably small number is not a violation of any law. Just like buying and selling a few guns - even if you make a little in the process - does not mean you violate laws requiring firearms dealers to have ATF licenses.
Federal law prohibits anyone except a licensed manufacturer from "engag[ing] in the business of . . . manufacturing ammunition, or in the course of such business, to ship, transport, or receive any ammunition in interstate or foreign commerce." (18 U.SC. section 922(a)(B)).
"Engaged in the business" means: "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." (18 U.S.C. section 921(21)(B)).
"With the principal objective of livelihood and proft" means "that the intent underlying the sale or disposition . . . is predominantly one of obtaining livelihood and pecuniary gain, as opposed to other intents." (18 U.S.C. section 921(22)).