Clayton Cramer has this video outlining his study on the effects of private transfer bans on murder rates:
I think it would probably make sense to control with states that didn’t pass private transfer bans. If murder rates are declining generally (as they were in the mid to late 1990s), or rising generally (as they were in the late 1960s), it would mask the true effect. I think it would be more accurate to say that, for instance, California’s murder rate increased faster than states that didn’t pass such a ban. Maryland and Pennsylvania are both listed as success stories, but during those periods in question, murder was falling generally.