Americans are nearly twice as likely to die from suicide by firearm as they are to be killed by someone else with a firearm, contributing to a rising suicide rate in this country, according to a new study published in the Annual Review of Public Health.

Suicides accounted for nearly two-thirds of deaths from firearms in 2012, according to the study, "The Epidemiology of Firearm Violence in the Twenty-First Century United States." That year, 20,666 people died from suicide by firearm, compared with 11,622 who died from homicide by firearm.

That's one firearm suicide every 25 minutes.

Mass shootings, such as the deadly school cafeteria shooting near Seattle in October, make headlines but account for a small percentage of firearms deaths each year, according to the study by Dr. Garen Wintemute, professor of emergency medicine and director of the Violence Prevention Research Program at the University of California, Davis. The four deadliest mass shootings in this century killed 84 people, the study says.

What makes guns such a common method of suicide?