Webster's team recently analyzed trace data from handguns used in city crimes from January 2006 through September 2015. After the state law went into effect, the researchers found, the number of guns used in a crime within a year of their retail sale in Maryland by someone other than the purchaser — which can be a sign of a gun diversion or straw sale — dropped by more than 75 percent, or from slightly more than three guns per month on average to slightly less than one.