In the FEDERAL FIREARMS REGULATIONS REFERENCE GUIDE 2005 published by the ATF:
(H4) Is a person who reloads ammunition required to be licensed as a manufacturer?
Yes, if the person engages in the business of selling or distributing reloads for the purpose of livelihood and profit. No, if the person reloads only for personal use.
[18 U.S.C. 922(a) (i) and 923(a), 27 CFR 478.41]
Interestingly, the GCA defines "ammunition" as (s.921(a)):
(A4) What kinds of ammunition are covered by the GCA?
Ammunition includes cartridge cases, primers, bullets or propellant powder designed for use in any firearm other than an antique firearm. Items NOT covered include blank ammunition, tear gas ammunition, pellets and nonmetallic shotgun hulls without primers. Generally, no records are required for ammunition transactions. However, information about the disposition of armor piercing ammunition is required to be entered into a record by importers, manufacturers, and collectors. A license is not required for dealers in ammunition only.
[18 U.S.C. 921(a)(17) and 922(b)(5), 27 CFR 478.11 and 478.125]
(B4) May an unlicensed person obtain ammunition from an out-of-State source?
Yes, provided he or she is not a person prohibited from possessing or receiving ammunition. [18 U.S.C. 922(g) and (n)]
(F11) Is a license required to engage in the business of selling small arms ammunition?
No. A license is not required for a dealer in ammunition only, but a manufacturer or an importer of ammunition must be licensed.
[18 U.S.C. 922 (a)(1)(B)]