In July 1974, Massachusetts adopted an amendment strengthening the criminal penalties attached to a longstanding law prohibiting the carrying of firearms without the appropriate permit and introducing a mandatory minimum sentence for every defendant convicted under the law. Unlike many other mandatory sentencing laws, this amendment explicitly prohibits the courts from interposing informal dispositions which might short-circuit the imposition of the new penalty (a minimum sentence of 1 year in prison without suspension, parole, or furlough). This study of the law's impact examined arrest summaries, ballistics unit records, statistics from the Department of Public Safety on the issuance of various firearm permits, survey data, and the results of interviews with attorneys, criminal justice officials, and offenders. In estimating the impact of the law on the use of firearms in violent crime, Boston police monthly incident reports were consulted. Prosecutions for firearms crimes were also examined. Findings show that most of the dire predictions about police, prosecutorial, and judicial evasion of the law have not proven accurate. The law has increased the likelihood that those accused of aggravated assault will be imprisoned. As a result of the mandatory minimum, judges were more likely to bind defendants over to superior court, and defendants were much more likely to appeal their convictions, resulting in a doubling of the proportion of cases requiring a superior court proceeding. The issuance of firearms licenses has dramatically increased, and the use of firearms in assaults has decreased in the year after the law went into effect. The appendixes contain tabular data and a detailed discussion of the study methodology. A total of 162 footnotes are provided.