From The Washington Post:

There are lots of means to perpetrate mass murder: with poison gas, with bombs, by running people over with trucks, working them to death in slave labor camps, or even by hacking them with machetes, as in Rwanda. However, mass shootings have been among the most common methods of mass murder around the world for more than a century. Even when victims are killed by other means, such as deliberate starvation or gas chambers, a government monopoly of arms is essential for governments being able to prevent the victims from resisting.