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When President Obama wept at a press conference earlier this month about gun control, his tears might have been born of frustration as well as sadness.

Despite frequent mass-shootings, he has struggled to substantially reform US gun ownership rules.

The organisation often credited - or blamed - for that is the National Rifle Association (NRA), a lobbying organisation with vast resources and a highly-effective grassroots base.

Four experts talked to the BBC World Service Inquiry programme about the NRA's rise to power.

Warren Cassidy: How the NRA became political

Warren Cassidy is retired executive vice president and chief executive officer of the NRA.

"The NRA was founded in 1871 shortly after the Civil War. Up through the first half of the 20th Century, it was known as a marksmanship organisation, a home for hunters and collectors.

"As political activity became more active in the US, when Jack Kennedy was shot, and then later Martin Luther King and Bobby Kennedy, this started a real political movement.

"We had to get active because legislation was appearing, and the 1968 Gun Control Act required more licensed dealers involved in the transfer of firearms. That sounds like a harmless thing, but it causes a great deal more trouble for the law-abiding public.

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"A few NRA directors were not happy with the sedate approach that a majority of the board were taking toward what we viewed as political threats. Some of them outright opposed us even dirtying our hands in the political arena.

"Then in 1977, we had the Cincinnati revolt. We came up with our own agenda, our own proposals on the floor of the annual meeting.

"We dismissed the current president, and went to the heart of the matter of the political action, and were successful. Many of us would say we became a political organisation at that point.

"Today, while the NRA is the leading educator of young shooters and conservation practices in hunting, it has tremendous [political] influence.

"Its great strength is our local state organisations - the California Rifle and Pistol Association, the Mass Rifle Association, Gun Owners' Action League and so forth throughout the 50 states - and these organisations are all volunteer. We send out representatives and help finance elections.

"We grade candidates who run in local and state offices on how they stand on the right to keep and bear arms, and follow them through their jobs as city councillors or mayors or governors and when they run for Congress.

"We spend the great majority of our money getting voters out. You can ask any one of those candidates elected to the Senate or the House of Representatives 'How did NRA help you?': 'Well, they got the voters out in my district or in my state'. That's where we work."

Professor Carl Bogus: How the NRA used the second amendment

Carl Bogus is a professor of Law at Roger Williams University in Rhode Island.

"The words of the second amendment read: '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', and what in essence it did was to say 'if the federal government doesn't arm the militia, the people can'.

"There had been only three US Supreme Court cases that dealt with the second amendment. They all held that the second amendment was related to militia service [and] granted a collective right, not an individual right. This was considered pretty settled until the 1960s.

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"There was a concerted campaign by the NRA to change this. They stimulated lots of writing - particularly in American law reviews - saying the second amendment should grant an individual right [to bear arms].

"They won the war in 2008 in a case called 'The District of Columbia versus Heller', when the Supreme Court held for the first time that the second amendment grants an individual right.

"The nine justices of the US Supreme Court divided five to four along perfectly ideological lines. The conservatives said it grants an individual right, and the liberals all said, no, it grants a collective right.

"The individual rights view promoted by the NRA became embedded within the modern conservative movement, and therefore the Republican party.

"The political obstacle [to gun control] is not the sense of the American electorate per se. Maybe 80 or 90 per cent of Americans favour more stringent gun control.

"But the intensity factor of a very small fraction of the electorate - it may only be one or two per cent - who will absolutely not vote for a candidate that favours gun control makes this a controlling issue."

Richard Feldman: NRA exploits guns' symbolic value

Richard Feldman is president of the Independent Firearm Owners' Association, and previously regional political director for the NRA.

"The best way to understand the NRA's incredible success is to take a step back. When we talk about guns, we're also talking about something else. Guns are a very symbolic issue, not unlike the flag.

"To millions of Americans, a gun is a symbol of all sorts of positive, traditional values of independence and freedom. When the government which can't protect its citizens wants to restrict the rights of citizens who have never misused their guns, those citizens get fearful.

Image copyright AP / Ralph Barrera Image caption To many Americans, guns represent freedom

"[The NRA] doesn't determine the outcome of an election that's 60/40, but the races that matter are on the cutting edge; 45/55. In an important year the gun issue can shift five per cent of the voters, making it a 50/50 race.

"There are few groups in this country that can actually reach 10,000 to 20,000 people in anyone's district, and every person represents more than one vote.

"I might, in meeting with a congressman, say 'Look, Congressman, I have two letters here, the letter that will go out to 15,000 of our members in your congressional district if you vote this way saying you're a fellow NRA member. The other letter, Congressman, reads pretty much the opposite'. Is that hardball? You bet.

"In 1994 [when President Clinton introduced a ban on assault weapons] I remember being asked 'Why do you need these guns?' My response was 'Well, I never needed them before, but if the government thinks I shouldn't be able to own them, I guess I want them now', and I did go out and buy about 15 of them before the ban.

"Every time our president or the Congress decides to limit and restrict some part of the firearm, boy, is there ever a run on guns. I absolutely can't believe the buying frenzy going on in this country as we speak.

"President Obama, whether he likes it or not, really does deserve the salesman of the millennia award: he is responsible for the sale of more firearms to more Americans than any person in history."

Professor Brian Anse Patrick: NRA has an effective media machine

Professor Brian Anse Patrick is an expert on American gun culture and teaches courses in propaganda and communication.

"The NRA is vastly effective, and it's augmented by what I call 'anti-media'. The NRA was the first group I knew of that went online, that had e-mail bulletins.

"More people read gun culture in forums and so on than read the New York Times. And the NRA itself prints three magazines it sends to different subgroups; one that's strictly political, one aimed at hunters, another aimed at people who just like to shoot. The NRA has approximately five million members who receive these magazines.

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"There are a lot of subcultures: target shooters, women and gun organisations, gay gun right groups. There's an organisation called Jews for the Preservation of Firearms Ownership. Their motto used to be 'For every Jew, a 22'.

"There used to be the 'I'm the NRA' campaign. They would show nice, wholesome people on a billboard - a female police officer, a game warden, a professional, an attempt to reverse this evil gun lobby menace that the New York Times loves to play with.

"If it weren't for the New York Times and its railing against gun culture, the NRA would not be in the position of strength it is in today. I looked at coverage from what we would call elite newspapers for a 10-year period: the more negative coverage the NRA received, the more members it received.

"There's a lot of reasons for this, one of which is that the gun culture has become what would be considered a classic social movement. Social movements tend to be organised around a sense of identity, like a grievance-based sense of identity. They're in conflict, and the resulting consequences of that are solidarity.

"Gun culture turned from this bunch of duck hunters to this annoyed mass of people."

The Inquiry is broadcast on the BBC World Service on Tuesdays from 12:05 GMT. Listen online or download the podcast.