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Six days after the Christchurch mosque attack, New Zealand Prime Minister Jacinda Ardern announced a ban on "military-style" semi-automatic rifles, prompting questions in the US.

Following a series of mass shootings in the US in recent years, there has been little in the way of sweeping gun-control reforms.

On the federal level, at least, the interest and attention in new legislation has led to almost no action in decades, despite numerous polls showing widespread public support for measures like strengthened background checks and banning certain types of high-capacity gun magazines and military-style assault rifles.

The Trump administration has issued a regulatory ban on bump-stock modifications that allow semi-automatic weapons to fire like machine guns, and there have been some tweaks to the background check database for gun-store purchases.

Last March, Donald Trump entertained the notion of more ambitious, "comprehensive" legislation, telling senators pro-gun lobbyists had little power over him. But there's been no such talk from the president since.

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Part of the reason New Zealand is ability to move quickly, of course, is that it's a parliamentary democracy, ensuring that the government is controlled entirely by one party or a politically compatible coalition. That's not the only explanation for why the US has charted a different course, however.

Here are five big obstacles that stand in the way of the kind of the US taking the kind of quick, major changes to firearm policy being advanced in New Zealand.

The NRA

Image copyright Getty Images Image caption A woman shows off a model gun and holster at an NRA fashion show

The National Rifle Association is one of the most influential interest groups in US politics - not just because of the money it spends on lobbying politicians, but also because of the engagement of its five million members.

It opposes most proposals to strengthen firearm regulations and is behind efforts at both the federal and state levels to roll back many existing restrictions on gun ownership.

In 2016 the NRA spent $4m on lobbying and direct contributions to politicians as well as more than $50m on political advocacy, including an estimated $30m to help elect Donald Trump president.

Its overall annual budget is roughly $250m, allocated to educational programmes, gun facilities, membership events, sponsorships, legal advocacy and related efforts.

Media playback is unsupported on your device Media caption A guide to the weapons available in the US and the rate at which they fire

More than just the numbers, however, the NRA has developed a reputation in Washington as a political force that can make or break even the strongest politicians.

It grades politicians on their votes and directs its resources and those of its membership - both financial and organisational - to supporting its fiercest advocates and defeating staunch opponents.

As one former Republican congressman told the New York Times in 2013: "That was the one group where I said, 'As long as I'm in office, I'm not bucking the NRA.'" Last March the president said he wasn't "afraid" of the NRA - but that was a rhetorical flourish that has not resurfaced.

Could it change? Gun-control groups, backed by wealthy benefactors like former New York City Mayor Michael Bloomberg, have become more organised in recent years, attempting to match the NRA's political might. They actually outspent the NRA in the 2018 mid-terms, during which some prominent pro-gun-control Democrats won close elections. The NRA's revenues dropped $56m in 2017, driven largely by a decline in membership dues. It still brought in $378.1m that year, however, ensuring that it will be the biggest single player in the firearm debate.

Gerrymandering

For the first time in eight years, Democrats have control of the House of Representatives - and their success in the 2018 mid-term congressional elections was fuelled largely by victories in suburban swing districts. In Atlanta, for instance, a gun-control activist won one of the closest races of the election, unseating an incumbent Republican.

Media playback is unsupported on your device Media caption What's Donald Trump said about guns and gun control?

Despite these advances for the party, the House electoral playing field is still tilted toward Republicans, who tend to be for gun rights. Due to the way the lines of House congressional districts are drawn, many by Republican-controlled state legislatures, there are more seats that tilt to the right than the left. While 2018 was a Democratic wave election, political gravity may eventually reassert itself, giving control of the chamber back to conservatives.

Demographics also play a part in the pro-gun sentiment in the House, as there are more rural districts with higher levels of gun ownership than there are urban ones. Racking up big pro-gun-control majorities in urban areas does little to change the political realities in the House.

Image copyright AFP Image caption House members sit on the floor "to demand action on common sense gun legislation"

For the moment, however, the initiative is with Democrats, and 2019 marked a significant step forward for gun-control proponents in Congress. One of their first actions was to pass a bill in the House requiring comprehensive background checks of private gun sales, including those that take place at gun shows. Previously the requirement to run the name of gun purchasers in a federal database was limited only to registered gun dealers.

Prior to 2019, attempts to pass new federal laws regulating firearms had been over before they ever really begin, stymied by House Republicans. In June 2016 a group of Democratic politicians staged a sit-in on the floor of the House to protest over the Republican House leadership's decision not to hold a vote on two gun-control bills.

Could it change? The House of Representatives was once one of the biggest obstacles to federal gun-control legislation. It no longer is - for now. With every House seat up for election every two years, and an American electorate that has proven fickle, the door could slam shut again with little advanced notice.

The filibuster

Now that gun-control bills have hopes in the House of Representatives, the Senate - where the rural-urban divide plays itself out on the state level - becomes the biggest obstacle to legislative success. States dominated by big-city voters, such as New York, Massachusetts or California, are outnumbered by rural and Southern states with pro-gun sentiments.

The rules of the Senate can also thwart efforts to enact more stringent firearm regulation, thanks to the "filibuster" - a procedural hurdle that means most major pieces of legislation need the backing of 60 out of 100 senators to pass, rather than a simple 51-vote majority.

In 2013, following the Newtown, Connecticut, school shooting, it appeared that efforts to strengthen gun-purchase background checks had significant bipartisan support in the Senate. After a concerted lobbying effort by the NRA, however, the bill received only 56 votes in favour, four short of the mark necessary to break the filibuster.

Media playback is unsupported on your device Media caption Barack Obama’s mass shooting speeches

No gun-control measure has come close to passage since then. At least for the moment, there appears little chance the Senate will take up the House-passed universal background check bill, despite indications that a majority of senators would vote in favour of it.

Could it change? Mr Trump has been a vocal proponent of doing away with the Senate filibuster, as he views it as an obstacle to enacting his legislative agenda. Several Democratic presidential hopefuls have made similar calls. A majority of senators are on the record against changing the rules, however.

The courts

Image copyright Getty Images Image caption Protesters in front of the Supreme Court in 2008

With Congress deadlocked on new gun legislation, left-leaning US states have taken a greater role in implementing gun-control measures.

After the 2012 school shooting in Newtown, Connecticut, 21 states passed new gun laws, including imposing assault weapons bans in Connecticut, Maryland and New York.

Some of the laws have run up against another barrier, however - the US judicial system. In recent years the Supreme Court has twice ruled that the right to own personal weapons such as handguns is enshrined in the constitution.

Media playback is unsupported on your device Media caption How countries around the world introduced restrictions following mass shootings

The Second Amendment says that "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".

Gun-control activists point to the introductory clause as evidence that the amendment was meant primarily to create a "well regulated" militia. In 2008, however, a sharply divided court held that the Second Amendment provides a broad right to firearm ownership that prohibits stringent registration requirement for personal weapons.

Since then, lower courts have considered challenges to state-imposed assault weapon bans, registration requirements and open-carry prohibitions.

Could it change? Trump-appointed Justices Neil Gorsuch and Brett Kavanaugh have a record of viewing Second Amendment rights broadly. The president is filling out the ranks of the lower courts with pro-gun-rights judges. If anything, the judiciary is moving to the right on this issue. In the autumn, the Supreme Court will consider a challenge to a New York City law restricting how handgun owners can transport their firearms. Gun-control advocates fear the high court is poised to strike another blow against state and local regulations.

The enthusiasm gap

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Perhaps the single biggest obstacle to new gun-control laws at the national level is that opponents tend to hold fiercely to their beliefs, while support for new regulation tends to ebb and flow around each new instance of violence.

The NRA's strategy, and that of pro-gun politicians, is to wait out the storm - to delay legislative efforts until attention turns elsewhere and the outcry fades.

Pro-gun politicians offer their thoughts and prayers, observe moments of silence and order flags flown half-staff. Then, in the quiet, legislative efforts are deferred and ultimately derailed.

The mass student-led protests following the Parkland school shooting focused white-hot attention on the issue for a time, but the marches have died down and the changes to gun laws, at least on the federal level, have been minimal.

Shortly after the Las Vegas shooting in October 2017, White House Press Secretary Sarah Huckabee Sanders told reporters "there's a time and place for a political debate, but now is the time to unite as a country".

Mr Trump said "we will be talking about gun laws as time goes by".

As time goes by. As that song from the film Casablanca says, it's still the same old story.

Could it change? According to one poll during the 2016 presidential campaign, guns were an important issue for both Democrats and Republicans. That could be a reflection of that year's mass shooting in an Orlando nightclub or the first indication of a new trend.

Follow Anthony Zurcher on Twitter.