No policy designed to prevent firearm violence is more popular than mandatory sentence enhancements for gun crimes. By providing stiff and certain penalties when a gun is involved in an offense, sentence enhancement laws' promise to reduce the use of firearms by criminals. Because the laws apply only when a crime is committed, they impose no direct costs on legitimate gun owners. Opinion polls find that a large majority of the public favors mandatory sentence enhancements, and more than half the states have adopted them.2 If these laws deliver their expected crime preventive effects, they are an especially attractive approach to regulating the use of firearms.