"(F7) May a licensee sell interchangeable ammunition such as .22 cal. rimfire to a person less than 21 years old? [Back]
Yes, provided the buyer is 18 years of age or older, and the dealer is satisfied that it is for use in a rifle. If the ammunition is intended for use in a handgun, the 21-year-old minimum age requirement is applicable. "
So you can buy ammo, but you have to convince the seller that the ammo is intended for use in a rifle. A good way to do this would be to bring the cased, unloaded rifle to the shop (not Walmart or similar) and ask for the correct ammo to match the gun.
Mind you this is FEDERAL law. Your state laws may vary.