Suicide is the tenth leading cause of death for all ages in the United States. Among those who attempt suicide, an estimated 96% fail. However, among those who attempt suicide with a firearm, a mere 17.5% fail (a fatality rate of 82.5%). This is much more lethal than methods such as drug/poison ingestion or cutting which have case fatality rates of 1.5% and 1.2%, respectively. Despite being one of the least used methods to attempt suicide, firearms account for more than half of completed suicides.
One of the most consistent findings in suicidology literature is that means matter that is, limiting access to effective ways of committing suicide is likely to decrease the probability of a successful attempt. Given that 90% of individuals who first fail a suicide attempt do not succeed in ever committing suicide over the course of their lifetime, we should expect suicide rates to decrease with declining levels of gun ownership, as suicidal individuals substitute away from guns to less effective means. Further, even if there is a 100% substitution rate in suicide attempts (a person who would have used a gun uses a different means), the completed suicide rate will go down due to alternative means being significantly less lethal.
The argument that suicidal individuals will find a way irrespective of the means available to them also ignores empirical evidence showing that most suicides involve very little preparation prior to the attempt, and are often impulsive decisions. One study found that 24% of individuals who attempted suicide took less than 5 minutes between the moment of decision and the suicide attempt. 70% of people took less than an hour. Given the impulsive nature of suicides, firearms are particularly unforgiving as they do not often afford a second chance.
On top of the strong theoretical foundation linking gun ownership to increased suicide rates, the overwhelming majority of case control studies done in the United States have shown that gun ownership is a strong risk factor for suicide, typically showing a 2 to 10 times larger risk of suicide for gun owners relative to non-gun owners. This finding holds true even after controlling for various characteristics that might be different between gun owners and non-gun owners such as demographic and psychopathological variables, suicidal ideation, and past attempts. Moreover, the increased risk of suicide to gun owners is not contained to the gun owner, but is shared by the spouse and children. As a recent Harvard study pointed out, there is no other plausible factor that could explain the significant difference in suicide rates between gun owners and non-gun owners.