The US government defines an active shooter as an individual actively engaged in killing or attempting to kill people in a confined and populated area. If you encounter this, the official advice is simple: run.

If running's not an option, hide. If hiding's not an option, you're advised to fight back with everything you have. But not all active shooters are the same, and new research reveals how one single factor effectively doubles the danger of a person who's trying to shoot you and everyone around you.

In a new study, researchers at Brigham and Women's Hospital in Boston sifted through FBI data and media reports on all active shooter incidents between 2000 and 2017.

Despite the heated, never-ending debate over gun control in the US, the researchers say there's currently no comprehensive scientific assessment on the kinds of injuries sustained by different types of firearms.

To provide one, they analysed records of all 248 active shooter incidents in the US over an 18-year period, leaving only two deadly episodes out: the Las Vegas shooting of 2017 (a death toll so high it was excluded for being a "statistical outlier") and the 2015 San Bernardino attack (since it was perpetrated by two active shooters, not one).

The data reveal that while semi-automatic rifles are only used in a minority of active shooter incidents (24.6 percent), those attacks result in almost twice as many people being wounded or killed compared to shootings that don't involve semi-automatic firearms.

"Active shooters are hell-bent on killing people," lead researcher and trauma surgeon Adil Haider told AP.

"The big difference – and this is not such a big surprise – is if you give them a semi-automatic, they're able to shoot twice the number of people."

That enhanced killing ability comes courtesy of semi-automatics' self-loading capabilities, which mean bullets can be quickly and repeatedly fired with every separate trigger pull and release, whereas single-shot firearms – including many – require manual loading, which is slower.

The tragic and undeniable truth is everybody knows this already; but we now have statistical proof of just how much more efficient semi-automatics are at killing innocent people.

On average, attacks with semi-automatic rifles wound 5.48 people per active shooter incident and kill 4.25 people per incident. By contrast, incidents with only non-semi-automatic firearms (including handguns, shotguns, and other kinds of rifles) wounded 3.02 people on average and killed 2.49.

"It's a very simple study," Haider told Bloomberg.

"What it shows is that if you have an active shooter incident, if the person has a semi-automatic, they're able to shoot twice as many people, and hence twice as many people end up dying."

Bullet for bullet, however, your chances of dying if you're struck are about the same no matter what kind of firearm your assailant uses – it's just that semi-automatics are better at spraying fire quickly, which makes it more likely you could be hit.

Of course, the bigger story is why we didn't know this – scientifically speaking, that is – before now.

The truth is, the US has a long, unfortunate history of not funding firearms research at the federal level, something which severely cripples our ability to understand American gun violence and how to enforce protections preventing it.

"There's just no support for gun research," Haider explained to Forbes.

"That's why you haven't seen a lot of this, and even these basic questions have not been answered."

In light of the amount of lives lost to fatal shootings in the US every day, this lack of research is a national tragedy, since studies show that in other economically developed nations without such restrictions on research, gun control actually works and saves lives.

That's a truth you can't run away from, no matter how much some people may try to hide it.

The findings are reported in JAMA.