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Firearms instructor Mike Magowan (L) works with student Debra Dunbar during a concealed weapons permit class at Take Aim Gun Range in Sarasota, Florida December 15, 2012.BRIAN BLANCO/Reuters

The debate over gun control has waxed and waned in the United States but now, following the Newtown school shooting, there are urgent new calls for tighter laws. Here are some the legal hurdles gun-control advocates have faced in the past:

Why do Americans believe that they have a right to have firearms?

Because of their Constitution's Second Amendment. This clause was ratified in 1791, when memories of the American revolution were still fresh and the former colonists believed that an armed citizenry was a bulwark against tyranny.

The amendment says: "A well regulated militia being necessary to the security of a free state, the right of the people to keep and bear arms shall not be infringed."

In a 1939 case (U.S. v. Miller), the U.S. Supreme Court decided that the Second Amendment applied only when there was "some reasonable relationship to the preservation or efficiency of a well-regulated militia." But the top court has since changed its tune. In two recent cases, it ruled that the Second Amendment referred to an individual's right to bear arms, not a collective right of a militia.

The court made that interpretation in two landmark cases that overturned local handgun bans, the 2008 District of Columbia v. Heller decision, then the McDonald v. City of Chicago case two years ago. According to UCLA law professor Eugene Volokh, this means some form of gun control could be proposed but the more stringent bans would be struck down.

So who bankrolls these court cases?

The Heller case was initiated by Robert Levy, a wealthy, libertarian lawyer who interviewed a number of prospective plaintiffs.

The plaintiffs in the McDonald case were four Chicago residents who were supported by a group opposing gun control, the Second Amendment Foundation (SAF). Both the SAF and the National Rifle Association have actively challenged other gun laws in court.

The NRA for example funded a court challenge against Illinois, the only state with a flat-out ban on carrying loaded, ready-to-use guns outside one's home. The Illinois law was overturned last week by an appeals court, just three days before the Newtown shooting.

Isn't there a national gun control law?

American gun-control statutes are a patchwork of federal, state and local laws. Which statute has precedence? Experts talk about the legal doctrine of federal pre-emption, which states that Congress has the authority to "occupy the field" and enact national regulations that pre-empt the states.

When it comes to federal gun laws, the U.S. Congress has chosen, however, not to fully pre-empt the states, allowing for independent regulatory regimes.

According to the Brady Campaign to Prevent Gun Violence, state laws vary widely from jurisdictions. California, for example, limits the acquisition of handguns to one unit a month, bans large-capacity magazines and wants to set up a long-gun registry. Arizona requires no state permit to purchase firearms and allows law-abiding citizens to openly carry a handgun.

The general rule when there are inconsistencies between two sets of laws is that individuals have to comply with the stricter (usually federal) law.

How strict are the federal laws?

Supporters of gun control say federal laws have their limitations too.

Look for example at two laws enacted under former president Bill Clinton .

The Violent Crime Control and Law Enforcement Act of 1994 banned 19 military-style assault weapons and certain high-capacity ammunition magazines of more than 10 rounds. It was championed by then-congressman and now Vice-President Joe Biden.

The law had a 10-year sunset clause on the provisions banning assault weapons, and the ban expired without further congressional action in September, 2004.

Another law, the 1993 Brady handgun Control Act, requires that federally licensed gun dealers conduct background checks of gun buyers. But it contains no such requirements for unlicensed sellers, meaning that anyone can walk into a gun show and purchase weapons with no questions asked.

The Columbine High School killers, Eric Harris and Dylan Klebold, used four firearms that came from gun show sales.