A study cited in the NIJ memo I linked earlier, to demonstrate that background check implementation didn't reduce actual homicide rates, assumedly the perpetrators simply moved to another market.
http://jama.jamanetwork.com/article....ticleid=192946
Compared states that didn't previously have background checks, and then got them via the Brady Law to states that already had background checks before hand.
Not that I'm against background checks, just pointing out that saying they stopped people from buying guns, is not the same as "they stopped criminals from getting guns".