Public support for a nationwide ban on assault weapons has jumped sharply after a gunman used a semi-automatic assault rifle to kill 49 people at a gay nightclub in Orlando.

According to a new CBS News poll conducted in the days following the Orlando shooting, 57 per cent of Americans now say they support a nationwide ban on assault weapons. That's up 13 percentage points from the 44 per cent support for a ban that the same poll showed in December.

The December poll showed the lowest level of support for an assault weapons ban in at least 20 years of polling. It was conducted in the aftermath of the mass shooting in San Bernardino, in which a man and woman used assault rifles to kill 14 people and wound 21 more.

The shooting in Orlando has reignited public debate over the proper role of military-style weapons in society.


Seven out of the eight latest public mass shootings have been committed with an assault rifle, according to a database of these shootings maintained by Mother Jones Magazine. The weapons can hold many rounds of ammunition - typically 30 - and skilled shooters can reload them in a matter of seconds. This makes it possible for a shooter to discharge many rounds of ammunition in a short period of time, killing or wounding dozens of people in a matter of minutes.

On top of this, assault rifles are typically more powerful and more accurate than other types of weapons. The AR-15, the most popular type of assault rifle available to civilians today, is accurate at up to 550m. Bullets fired from the gun leave the chamber at a speed of 975m per second, or a little over 3220km/h, according to one manufacturer.

By contrast, Glock 9mm pistols, one of the more popular handguns on the market, typically fire bullets with a muzzle velocity between 300 and 457m per second, or between 1000 and 1600km/h. And handguns are typically far less accurate than assault rifles, with maximum accurate ranges between 50 and 100m depending on the shooter.

Gun control advocates question the wisdom of putting powerful assault rifles, like the ones used in Orlando and San Bernardino, in civilian hands.

These "weapons of war" were designed specifically for killing people on battlefields. Gun manufacturers often use the weapons' military heritage as a selling point, according to a 2011 report by the Violence Policy Centre, a gun control research and advocacy group.

Gun rights groups point out that the rifles are rarely used in the commission of crimes. They further argue that bans on the weapons wouldn't work since there are already millions of them in circulation.

However, hundreds of thousands more assault rifles are manufactured and sold each year. A ban could help prevent a would-be mass shooter from walking into a gun store and walking out the same day with a brand new assault rifle - which is what the Orlando shooter did just a few days before he committed the worst mass shooting in modern US history.

Meanwhile, a study has found that for every gun used in self-defence, six more are used for crime.

Last December a woman in Florida was awakened by the sound of an intruder in the house. She grabbed her gun, confronted the intruder and fired one fatal shot, according to police.

This is the type of scenario many Americans envision when buying their first or second or third firearm.

Forty-eight per cent of gun owners say they own a firearm primarily for protection, according to the Pew Research centre, a share that's up 22 percentage points since the late 1990s.

Gun rights groups like the National Rifle Association compile reports of incidents like the Indianapolis home invasion to point out the social benefits of good guys with guns who protect themselves and their communities from would-be wrongdoers.

But the latest research on the prevalence of firearm use in self-defence finds that these incidents are much less common that many gun rights advocates believe. For every person who uses a gun in self-defence, the research finds, nearly six people use a gun to commit a crime.

Those figures come from a Harvard University analysis of data from the federal National Crime Victimisation Survey. David Hemenway, director of the Harvard Injury Control Research Centre, examined five years of data from the survey covering the period between 2007 and 2011, with responses from nearly 160,000 individuals.

Hemenway found that not only are self-defence gun uses rare - people defended themselves with a gun in roughly 0.9 per cent of crimes committed over this period - but in many cases they don't lead to better outcomes for crime victims.

"The likelihood of injury when there was a self-defence gun use (10.9 per cent) was basically identical to the likelihood of injury when the victim took no action at all (11.0 per cent)," Hemenway and co-author Sara Solnik found.

Looking at what happened after people took action to prevent a crime, Hemenway and Solnik found that people were far better off either running away, or calling the cops if possible, rather than attempting to stop a crime with a gun. "Running away and calling the police were associated with a reduced likelihood of injury after taking action; self-defence gun use was not," they write.

The use of guns were, however, were linked to better outcomes in certain circumstances. In cases of robbery and burglary, victims who didn't try to stop the crime lost property nearly 85 per cent of the time.

Victims who attacked the intruders or threatened them with a gun had a better outcome, losing property 39 per cent of the time. But those are slightly worse odds than for victims who fought back with other weapons - this latter group lost property 35 per cent of the time.

Of course, there's a huge problem with this analysis, as with all attempts to quantify and describe self-defence gun uses: These cases are so rare that it's difficult to gather a meaningful body of data about them. And conclusions based on analyses like these are subject to a high amount of error.

Among the 160,000 people surveyed in this dataset, for instance, there were only 127 instances where someone used a gun in self-defence. This kind of rarity leads to wild variation in estimates of how many times people use guns in self-defence in a given year.

One of the most famous estimates of the annual prevalence of self-defence use, made in the mid-1990s by criminologists Gary Kleck and Marc Gertz, claimed between 2.2 and 2.5 million defencive gun uses annually. This estimate was based on a national survey of 5000 people, of which 66 reported using a gun in self-defence in the past year.

But subsequent research has found that this high number is simply mathematically impossible. As crime prevention researcher Philip Cook has pointed out, "The Kleck-Gertz survey suggests that the number of DGU [defencive gun use] respondents who reported shooting their assailant was over 200,000, over twice the number of those killed or treated [for gunshots] in emergency departments." In other words, in order for the estimate of 2.5 million defensive gun uses to be correct, we would have to assume that self-defence accounts for literally every single gunshot victim in the United States, as well as a massive number of invisible gunshot victims completely unknown to medical or legal authorities. That simply isn't plausible.

In an email, Cook notes that getting reliable survey data on rare occurrences can be tricky. "Whenever you're surveying about a rare event like DGUs, estimates may well be inflated by the small fraction of respondents who are drunk or deluded or simply having fun," he said. He points out that the percentage of people reporting a defensive gun use in Kleck's survey is similar to the percentage of Americans who say they've been abducted by aliens.

A more reasonable estimate, based on the National Crime Victimisation Survey, would peg the annual number of self-defence gun uses to be around 100,000 per year. Researchers generally view these estimates as more reliable because the NCVS includes a much larger sample size and it surveys the same households multiple times, which ensures that people are recalling events more accurately.

To be sure, 100,000 is still a very large number. But there's another problem, too, and that's that there's a lot of murkiness around what "self-defence" really means. "Self-defence is an ambiguous term and whether one is a defender or a perpetrator may depend on perspective," Hemenway and Solnik write in their latest research.

In a 2000 study, Hemenway and colleagues asked criminal court judges to read 35 accounts of gun owners who said they used their guns in self-defence in a national survey. In the judges' opinions, over half of these gun uses were probably illegal.

For instance, in one of the cases a 58-year-old man said that he was watching TV when a friend interrupted him. The man, who was carrying a gun at the time, told the friend that he was going to shoot him, and the friend ran outside. In the eyes of the man with the gun, this was an act of self-defense against a "verbal assault" by his friend.

In another, a man responded to an alarm at his business. When he arrived, he shot at two men standing outside the business without knowing whether or not they had set off the alarm.

In both cases above, men who said they were acting in self-defence were actually committing crimes of aggression using a gun. The simplistic narrative of "good guys" versus "bad guys" is often inadequate to describe the complicated interactions between people and their firearms.

The home invasion shooting in Florida is another case in point. The "intruder" was actually the woman's 27-year-old daughter.