In the United States, there are more firearms than people. Conversations around how to make schools and public places safer often call for stricter gun laws, but it would be impossible to make nearly 400 million weapons disappear — either overnight or even within 30 years. So if we are serious about preventing gun-related violence, we need to focus on deterrence and protection. After all, the root cause of any mass shooting is a person’s desire and willingness to harm others.