Firearms move from states with weaker gun laws to those with tougher restrictions where they fuel crime.  Interstate gun trafficking flourishes because states regulate gun sales differently and there’s no federal limit on the number of firearms an individual may buy at one time. Jurisdictions with weaker gun laws draw gun traffickers who purchase several guns and resell them in areas with tougher laws. According to FBI Reports (2010), 126 crime guns recovered in western Massachusetts between 2006 and 2010 were traced to Vermont.

Source:

Douglas S. Weil & Rebecca C. Knox, EFFECTS OF LIMITING HANDGUN PURCHASES ON INTERSTATE TRANSFER OF FIREARMS, JAMA, 1759, 1759-60 (1996).