The law intends to decrease the use of firearms in violent crimes by making a minimum 5 years of imprisonment mandatory for certain crimes where there was visible possession of a firearm, a prior conviction of the defendant within the previous 7 years, or commission of the crime in or near public transportation facilities. Analysis of the commission rates for armed robberies and armed aggravated assaults in 1981 (prior to the new law) and 1984 (after the new law) showed a large decline in the use of guns for both these offenses. Mandatory sentencing probably reduced the incidence of gun robberies somewhat below what it would have been without the law. The analysis suggests, however, that some of this reduction may have shifted to nongun robberies. The law had less of an apparent impact on the use of guns in aggravated assaults. Court workload and delay showed an increase for both offenses, and the length of detention for both offenses increased. 4 tables and 4 figures.