Guns

Background checks won't stop someone who has no history of violence from deciding to have one.

Killing a person shouldn't be easy.  A trigger makes it easy.

Someone who is thinking about killing other people should have to think about the flesh, bone and blood of those people, and about what it means to permanently suspend a human life.  A potential killer should have to think about the transformation of a moving, emotional being into less than the sum of its parts.