The Bartley-Fox Law, enacted in Massachusetts in April, 1975, was intended to reduce the incidence of gun related crimes and illicit carrying of firearms. Investigation of crime statistics to assess the effect of the law requires the isolation of exogenous factors and assessment of the displacement effects as independent variables. A modeling system was used to analyze FBI uniform crime reporting and Boston Police Department crime statistics on gun related and nongun related armed assault, armed robbery, and criminal homicide for Boston, the rest of Massachusetts, and 98 control communities. The analysis cover the period before Bartley-Fox, the period of public attention before actual implementation using annual figures. The results indicate a shift from more serious to less serious forms of criminal activity after the enactment of the law of from the 2-month publicity campaign that preceded it. Gun assaults with battery and assault-precipitated gun homicides show the greatest proportional reductions. However, increases in nongun related armed assault and robbery offset the gun related decreases by almost a 2 to 1 ratio. The effect of desegregation could not be determined quantitatively. The displacement effect in criminal homicide, on the other hand, was relatively minor, but data collection was limited to Boston and its control jurisdictions. Recommendations for further research are made.