So says the L.A. Times. But the National Crime Victimization Survey for 2005 (PDF page 81) reports that handguns are used in fewer than 8% of all crimes of violence. This doesn't include homicide, but homicides are fewer than 0.5% of all crimes of violence, so even if one includes homicides the answer would be about 8%.

Here's my guess about what happened: The petition does say that "Handguns are the weapon most likely to be used in a street crime," a statistic that the NCVS supports. According to NCVS, handguns account for a plurality (roughly 1/3) of weapons used in the about 25% of violent crimes that the victims believed were committed with weapons (as opposed to hands and fists), which is more than any other class of weapons. The petition's statement is thus accurate, though not terribly helpful and potentially misleading. But this then got transposed into the off-by-a-factor-of-6 L.A. Times statement that "[D.C. officials] argue handguns are involved in most violent crime."

Now I should stress that handguns are indeed involved in many crimes, and the majority of homicides. I oppose handgun bans because I think they do more harm than good, but I certainly don't want to pooh-pooh the harm done by criminals with handguns. But this sort of story helps show how skeptical one should be of media reporting about statistics (and likely about other things as well).

Thanks to commenter Tom Hynes for pointing this out.