This applies more to a research methods class (Colbert expresses a need for a control group in Copland's research. His suggestion? Sugar guns as well as a second experimental condition in which EVERYONE is given a gun). However, I imagine that you could show your students this video and pause it before they introduce the research project and ask your students how we could finally answer this question of whether or not gun ownership lowers crimes.
Thanks to Chelsea for pointing this out!