Few clarifications...
There is no federal law prohibiting a person under the age of 21 (and over 18) from buying a handgun in a private party sale; federal law only prohibits FFLs from selling handguns to that age group.
Correct.
Another way to say it: Under federal law, a person between the ages of 18 and 20 can legally buy, receive, own, and possess a handgun; it's just not legal for a FFL to sell one to him or her.
Yes he can transport it unloaded in a case unless your state dictates otherwise.
FWIW federal law actually doesn't address the manner in which the firearm is transported
unless one of the following circumstances applies:
- The firearm is being transported through a state where possession of the firearm is illegal under state law, but the firearm is legal to possess in the origin and destination states;
- The firearm is being transported through a school zone by someone who isn't licensed by the state to carry it, isn't a LEO, isn't a school employee, or doesn't have permission; OR
- The firearm is being transported by a juvenile.
In most of those cases (but not all of them!), the firearm is supposed to be unloaded and in a case.
Some one over 21 has to go buy the ammo.
The law is a good deal more subtle than this; it's actually somewhat of a "Don't Ask, Don't Tell" policy. If the buyer is under 21, the law is
specific to the firearm in which the ammo will be used. IOW it's legal for a FFL to sell .357Mag to an 18-year-old who's going to use it in a Marlin 1894 rifle, but it's
not legal for them to sell .30-30Win for use in a T/C Contender pistol. Yes, this is somewhat baffling and stupid, but I didn't write the law!
There's no legal requirement for the FFL to
actually ask the buyer what type of firearm he/she is using, so a knowledgeable clerk at a LGS can sidestep the law by simply not asking questions. After all, almost every commonplace pistol cartridge can be used in some sort of rifle, so the legality is easily explained away ("...I figured he/she had a Hi-Point carbine, I didn't have any reason to believe he/she had a pistol").
This was discussed at great length in the following thread (you can obviously ignore what it says about NJ):
http://thefiringline.com/forums/showthread.php?t=455180
Some big-box stores- notably one with the initials "WM"- won't sell pistol-caliber ammo to people under 21, but it isn't because federal law is stopping them; it's a private store policy intended to cover their rear ends if their ignorant minimum-wage clerks don't understand the law.
Mandatory disclaimer: I am not an attorney, nor do I play one on TV. This is not official legal advice. Caveat emptor and YMMV. 