In 2014, 29 people were killed in an attack with knives (multiple attackers). In 1927, 38 children were killed in a school bombing in Michigan. There was a sarin gas attack in Japan that killed many. There are also numerous examples where many have been killed by fire.
It's naive and wrong to assume that there would be no mass killings without guns. There would likely be less, but it wouldn't be 0. Before anyone counters me with "we should prohibit all guns because it will prevent some mass killings", we still have 2nd amendment.
