Mass killing has been done with fire, knives, rocks, bows. If someone is going to go off the deep end then they will find a way to do it.
Even governments do not do mass killings of their own people with bullets. The Nazis made use of starvation, disease, and fire. Mao Tse Tsung sent over 60 million of his countrymen to starve in the early 60's. Stalin about 20 million. They didn't need bullets then. But if the populace had been armed sufficiently, history of that part of the world would have been a lot different.
But your right. Lets stay on topic. If you see Pistol Marks, be sure to bring Ammo... And Viagra.