The Michigan Felony Firearm Law imposed a 2-year mandatory sentence for possessing a firearm while committing a felony and prohibited parole, probation, and suspended sentences. As a matter of policy, the Wayne County prosecuting attorney refused to drop charges under the gun law in exchange for a guilty plea. This study first used multiple regression analysis on over 8,000 cases disposed of by the Detroit courts during 1976, 1977, and 1978 to determine the gun law's impact. While the prosecutor did adhere to the policy of not reducing gun law charges, there was little change in the certainty or severity of sentences that could be attributed to the new law. Time series data was then collected on homicides, robberies, and assaults for 8 to 10 years before the gun law and 2 years following its implementation. Analysis results again failed to establish any strong relationships between the gun law and declining crime rates. Homicide with a gun was the only offense that experienced a statistically significant decline after implementation of the gun law, but examination of the assault data suggests that this trend was related to other factors. Although mandatory sentences for crime committed with a gun are politically popular because they do not affect legitimate gun owners, the Detroit experience suggests that the benefits of this approach are more apparent than real and may actually divert resources from more effective policies. Tables, charts, about 30 footnotes, and a description of the time series analysis are included.