The United States is once again debating whether it should follow the example of other countries in making it harder to buy a gun.

President Obama has renewed his call for tighter gun control laws in the wake of last week's San Bernardino, California shooting that killed 14 people.

Two countries radically changed their laws in the wake of home-grown massacres: Australia and the United Kingdom. Other countries have more relaxed laws. It's much easier, for instance, to buy and use a gun in Austria and Switzerland, than in most of Europe.

So how hard is it to legally to purchase firearms around the world?

Australia

To get a license, an applicant must state the reason for the firearm, whether for hunting or collecting. Self-defense is not a valid reason. Applicants must take a safety course before being issued a license. There is a 28-day waiting period.

That was introduced after 35 people were massacred in Port Arthur by a legal gun owner in 1996. Australia undertook a massive crack down on guns. Firearm laws had varied state to state so the national government imposed unified licensing, storage and usage laws throughout the country.

It standardized the definition of what is a "genuine reason" that an applicant must show for owning, possessing, or using a firearm. And it banned the import of most semi-automatic, self-loading and pump action firearms.

A shooting in Melbourne, Australia, in 2002 by a legal owner of handguns led to a crackdown on handguns to mirror the laws on rifles.

The proportion of homicides in which guns were used fell by half between 1990 to 2010 to 12%, and firearm suicides dropped by 65%, according to a study for the U.S. Library of Congress.

In 2012, there were 2.75 million registered firearms in Australia held by more than 730,000 individual license holders.

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The United Kingdom

"Great Britain has some of the most stringent gun control laws in the world," concludes the Library of Congress. Massacres in 1987, 1996 and 2010 were all perpetrated by lawful gun owners.

A gun license will only be granted if an applicant has a "good reason." Again, self-defense is not considered a valid reason. Generally, a license must be renewed every five years. All "military-style" firearms are banned, and handguns are nearly impossible to legally buy or possess.

Laws on shotguns are more relaxed. You can get a shotgun license for "profession, sport or recreation, or shooting vermin," and hence the guns prove popular with hunters on private land. They must be registered with local police, who can take as long as they like to consider the mental health of an applicant.

Only recently has the U.K. introduced a minimum age of 18 for a shotgun license to comply with EU law. People can shoot at gun clubs without a license under a long list of rules.

In 2012, there were 142,000 firearm certificates and over 560,000 shotgun certificates.

Austria

Austria is considered to have the most relaxed laws in the European Union.

From aged 18, Austrians can freely buy and own certain types of shotguns and semi-automatic rifles for sport and hunting. They only have to be registered with a licensed dealer within six weeks of purchase.

Non-repeating shotguns don't need a license.

Austria is one of the few EU countries where self-defense is a valid reason for gun ownership. Media reports say gun ownership in Austria has soared in the wake of the refugee crisis, which has seen hundreds of thousands of migrants from Syria and Iraq arrive in Europe.

Still, automatic weapons are banned, following the minimum standards which apply across the EU.

Switzerland

Based on tradition, Switzerland has unusual gun laws for Europe. While not a member of the European Union, it subscribes to EU regulations -- such as a minimum license age of 18 and a ban on automatic firearms and armor-penetrating ammunition - but there is a traditional right to bear arms and reservists are encouraged to keep rifles at home. A referendum to enact stricter gun laws was voted down in 2011.

Self-defense is a valid reason to apply for a license. A license is required if the gun is purchased from a dealer, but not if bought privately.

Related: These countries have tried to stop mass shootings

Canada

Canada has a ban on assault rifles and "military" style firearms like the AK 47. A firearm license is required for other guns. A safety test must be taken, and a third person character reference must be obtained.

There is a mandatory background check which includes "criminal, mental health, addiction, and domestic violence records," notes the Library of Congress. Licensing and registration is handled on the national level and must be renewed every five years.