Why do so many people believe that 21 is the minimum age to purchase a handgun?
Is it because they misapply the federal age restriction placed upon federal licensees in 18 USC § 922(b)(1) to everyone else?

(b) It shall be unlawful for any licensed importer, licensed
manufacturer, licensed dealer, or licensed collector to sell or
deliver -
(1) any firearm or ammunition to any individual who the
licensee knows or has reasonable cause to believe is less than
eighteen years of age, and, if the firearm, or ammunition is
other than a shotgun or rifle, or ammunition for a shotgun or
rifle, to any individual who the licensee knows or has reasonable
cause to believe is less than twenty-one years of age;

I understand that some non-licensees will be similarly restricted by their resident state law, but in most states the age is 18 which matches the minimum age defined by federal law for everyone else in 18 USC § 922(x):

(x)(1) It shall be unlawful for a person to sell, deliver, or
otherwise transfer to a person who the transferor knows or has
reasonable cause to believe is a juvenile -
(A) a handgun; or
(B) ammunition that is suitable for use only in a handgun.

…(5) For purposes of this subsection, the term "juvenile" means a
person who is less than 18 years of age.

(Ref: http://trac.syr.edu/laws/18/18USC00922.html for the full law)

So in most states, under 21 handgun purchases from non-licensee private sellers are not only legal but quite common. So why do you think so many people still claim it’s a federal crime?

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i am going to get a shotgun soon so i need to know some laws about. i know you cannot driving your car wtih a loaded gun inside. what’re some other laws i need to know?

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Legislation to require a federal license to possess any detachable-magazine semi-automatic rifle or shotgun, or any handgun, has been introduced in Congress. Bills to re-impose the federal "assault weapon" and "large" magazine ban, or to impose a much broader ban, have been introduced in Congress since 2003, and will likely be introduced in the current Congress soon.

Already, the deliberate deceptions we heard from anti-gunners previously are resurfacing. Anti-gun Sen. Carl Levin, D-MI, said last Thursday on the floor of the Senate that "assault weapons" are "capable of firing up to 600 rounds per minute" and that they are "once again pervading our streets and neighborhoods."

Did we mention that our opponents are deliberately deceptive?

Many fully-automatic firearms can fire 10 rounds in a second, which theoretically would work out to 600 rounds per minute, but they cannot be reloaded fast enough to achieve anything near that rate in reality. But we are not talking about fully-automatic firearms—we’re talking about semi-automatics, and the difference between them need not be explained here.

"Pervading our streets?" Anti-gun lawmakers swore up and down that once the "assault weapon" ban expired, the murder rate would go through the roof. Well, the ban expired in 2004 and since then, the murder rate has gone down to a 43-year low.

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How do armored car guards get a license to openly carry rifles or a shotgun in their truck and always have a holstered handgun on them at all times when no private citizen is allowed to get a permit? And fyi Illinois is a no-issue concealed carry firearm state.

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Gun Control, law concerning limits on the possession and use of firearms. All countries have restrictions on some firearms, which vary according to the history of the nation and its circumstances: in the United States, for example, its constitution provides for the right to bear arms. Though American legal scholars dispute the meaning of the constitutional provision, it has been used—together with a long history of private gun ownership—to resist strict firearms control in the United States. European countries have stricter laws. In the United Kingdom the police are generally unarmed and guns are seen to have little place in civilian life, except for some leisure purposes.

In the United Kingdom it is an offence to possess a firearm or a shotgun without a certificate from the police. Firearms certificates are granted if the applicant has good reason to have one, and presents no danger to public safety or the peace. People who have been sentenced to three years or more in prison, and those of intemperate habits or unsound mind, may not be granted certificates; a certificate may also be refused on any other reasonable grounds. Shotgun certificates are more easily obtainable, but the applicant still needs a good reason for having one: they are most usually used in shooting sports. An applicant refused a certificate may appeal to the Crown Court.

Certain particularly dangerous firearms are prohibited, such as most rifle weapons with automatic function or loading, all kinds of explosive or noxious munitions, and disguised firearms. Authority to hold such weapons may be granted by a government minister, most commonly to those involved in theatre and cinema productions, for signalling flares on ships and aeroplanes, or to certified museum curators. Significant restrictions are placed on their movement and storage.

There are a number of weapons for which no certificate is required, such as starting pistols for athletics, miniature rifles on fairgrounds, and air weapons up to a certain power. In certain circumstances it is not an offence to use a shotgun lawfully held by somebody else, for example, when shooting on private property.

Other firearms offences that may be committed regardless of whether the offender holds a certificate or not include carrying a loaded shotgun or air weapon in a public place, and carrying a firearm together with ammunition, whether or not the firearm is actually loaded. Shortening the barrel of a shotgun is illegal: “sawn-off” shotguns are a favoured weapon of bank robbers. Dealing in firearms is only permitted by a licensed dealer, who must have

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