Guns are the catalytic component in murder-suicide. Just as important, it must be understood that the emotional factors that drive suicide can be all too easily turned outward on friends, family, co-workers, and complete strangers because of the unmatched lethality of firearms. Every major murder-suicide study ever conducted has shown that a firearm—with its unmatched combination of lethality and availability—is the weapon most often used to murder the victims, with the offenders then turning the gun on themselves.

Josh Sugarmann
VPC executive director
August 5, 2002
http://www.vpc.org/press/0208nc.htm

[Apparently Mr. Sugarmann didn't include murder-suicide studies done in Japan in his review of papers.  Murder-suicide is far more common in Japan than in the U.S. and yet guns are almost never used.--Joe Huffman]