ATF P 5300.15 — Federal Firearms Licensee Quick Reference and Best Practices Guide

Age Restrictions Page 5
As noted above, under Federal law, the minimum age to purchase firearms and ammunition from an FFL is 18. If the firearm is other than a rifle or a shotgun—or ammunition for other than a rifle or a shotgun—the minimum age is 21 [18 U.S.C. 922(b)(1)]. However:

1. You may sell ammunition that is interchangeable between rifles and handguns to a buyer who is at least 18 years of age if you are satisfied that he or she will use the ammunition in a rifle.

2. Regardless of less restrictive State and local age requirements for firearms and ammunition purchases, you must adhere to the above Federal mininum age provisions.