Hi James
Existing definitions are not consensual. The FBI definition for mass murder includes 4 or more victims, that may include the perpetrator, during a very brief time period. Why 4 victims, why not 2, 3 or 6? Such arbitrariness suggests a probable reason why this definition plays no part in relevant legislative documentation in the USA.
Definitions for mass shootings are even more arbitary. Examples from authoritative American sources include examples where no death occurred.
Whilst you're correct that mass murders can occur without the use of firearms (e.g., the Boston Marathon bombers, who killed 3 people), because of previous postings, my comments about 'opportunity' were restricted to the USA. I should perhaps have made that clearer. As a proviso to the following, note that homicide data from the USA is less restrictive than that of some other countries. For example, US data includes some cases without a formal legal decision that the death was homicide rather than manslaughter, self-defence, accidental, etc. The most recent FBI data from 2016 reports that of 15,070 homicide victims, 11,004 (73%) were killed by firearms and 3,966 (27%) by other means. These numbers translate into rates of 3.4 homicide victims by firearms per 100,000 American residents and 1.2 victims per 100,000 residents by other means. Clearly, most US homicides were by firearms. You're also correct that more firearm deaths were classified as other than murder than as homicide (i.e., 7.4 versus 4.5 deaths per 100,000 people). The former include suicides and accidents.
The main point I was trying to make is that easy access to semi-automatic weapons, coupled with cultural factors and individual traits, suggests that correction for increasing numbers of mass murders in the USA is unlikely to occur soon.
Cheers, Mike