Federal law.
18 to buy a rifle or shotgun, 21 to buy a handgun or assault rifle.
18
18
18
Generally speaking, you have to be 18 to buy rifle or shotgun ammo, and 21 to buy handgun ammo.
18 in the US
18
Have never heard of an underwater assault rifle. In the US, you must be 18 to buy a rifle, and 21 to buy a fully automatic rifle.
You can buy a semi automatic Title I rifle at 18. To purchase a true, select fire (auto or burst) NFA tax stamped assault rifle, you must be 21.
Be 18, have money and no criminal record.
18.
You must be 18 to buy a rifle. The stock has nothing to do with it.