Researchers have found time and time again that more guns mean more deaths. And Americans have a lot of guns, and easy access to them. Americans own almost half of the 650 million civilian-owned guns there are in the entire world, and gun homicide rates in the US are 25 times higher than in other high-income countries. States and developed countries with more guns have more gun deaths.

Analyzing data on hundreds of shootings in Boston from 2010 to 2014, Anthony Braga of Northeastern University and Philip J. Cook of Duke University found that on a bullet-per-bullet basis, shootings committed with a large-caliber firearm are much more likely to result in a fatality than those with a smaller-caliber gun. Caliber is a measure of the diameter of the bullets fired by a particular gun.

But they found stark differences in shooting outcomes depending on the caliber of gun used. They divided the calibers of guns used in the shootings into three categories: small, which included .22-, .25- and .32-caliber handguns; medium, including .380s, .38s and 9mms; and large, including .40s, .44 magnums, .45s, 10mms and 7.62 x 39s.