Want more proof that gun laws just exist but do not make sense?
You need to be 21 under federal law to buy handgun ammunition.
You need to be 21 to buy a handgun through an FFL.
You do not need to be 21 to own a handgun, carry a handgun (where allowed by state law) or shoot a handgun, yes with handgun ammunition.
A parent or other private party can give or sell someone 18-20 a handgun in many states and it does not conflict with federal law.
So there is in fact 18-20 year old individuals who legaly have a permit to carry concealed in states requiring a license/permit/tax, or can carry a handgun concealed in states not requiring a Concealed carry license like Vermont or can even open carry in many more states that allow that.
So they can own, shoot and carry loaded handguns but they cannot legaly buy handgun ammunition for the purpose of using it in a handgun. So they can legaly have the handgun ammunition and the handgun, they just cannot legaly purchase the ammunition themselves.
Lying that it is for a long arm when it is intended for a handgun to simply bypass this idiocy would also be a crime as it is intentionaly violating the federal statute.
Welcome to the world of gun laws. Leave logic at the door.
edit to add someone in Vermont can even carry concealed with no permit if they are merely 16 and over. They however cannot buy ammunition for the firearm they can legaly carry loaded and concealed for another 5 years.