“There have been 135 mass shootings so far this year in America. It’s barely April. Ban assault weapons now.”

That was the claim from former Democratic presidential candidate and US Secretary of State, Hillary Clinton.

It looks like she’s using stats compiled by the Gun Violence Archive (GVA), which had recorded 135 “mass shootings” so far this year at the time Mrs Clinton tweeted.

How do you define ‘mass shooting’?

But what counts as a mass shooting is hotly disputed.

The GVA tally includes any incident in which four or more people were injured, whether or not anyone died.

If we restrict the count to only those incidents where there was at least one fatality and three non-fatal injuries, the number of mass shootings in the archive falls from 135 to 86 so far this year.

But the GVA is not the only source of mass shootings data.

The magazine Mother Jones compiles its own list and has more stringent criteria. By its definition, an event only becomes a mass shooting if three or more people die.

By Mother Jones’ estimate, there have been four mass shootings in the US so far in 2023. Those are the school shootings in Nashville and Michigan, as well as the two that struck around Lunar New Year celebrations in California.

Would banning assault weapons reduce mass shootings?

The second part of Mrs Clinton’s tweet urges a ban on “assault weapons”.

There’s no settled definition of this either, though in 1994, the US Department of Justice described them as “semiautomatic firearms with a large magazine of ammunition that were designed and configured for rapid fire and combat use”.

It’s not clear how many of the “135 mass shootings” Mrs Clinton refers to involved assault weapons. However, all four of the incidents in the Mother Jones database this year did involve semiautomatic firearms, according to reports.

Some gun control campaigners argue that assault weapons allow shooters to fire more bullets in quick succession, therefore increasing the possible death toll.

And a 2019 peer-reviewed study found that “mass-shooting related homicides” were lower between 1994 and 2004, when a ban on assault weapons was in place across America.

But as FactCheck has previously reported – and as the researchers themselves note – the evidence is “observational”. In other words, it can’t tell us whether it’s the assault weapons ban that caused the reduction in shootings and deaths.

And another major study of thousands of scientific papers by the Rand Corporation found “inconclusive” evidence that such bans lead to fewer mass shootings.

Indeed, assault weapons are only used in 16 per cent of mass shootings, according to gun control campaign group, Everytown. By contrast, handguns are the predominant choice for those carrying out a mass shooting, which suggests that limiting access to those firearms might be a better use of campaigners’ time.

FactCheck verdict

Hillary Clinton said there have been 135 mass shootings in 2023 already. This is supported by data from the Gun Violence Archive. However, that organisation uses a broad definition of what counts as a mass shooting and includes incidents in which there were no fatalities. A more stringent definition – at least three deaths – is used by the website Mother Jones, which has recorded four mass shootings this year.