Increasing criminal sanctions may reduce crime through two primary mechanisms: deterrence and incapacitation. Disentangling their effects is crucial for optimal policy setting. I use sentence enhancements due to the introduction of state add-on gun laws to isolate the deterrent effect of incarceration. Using cross-state variation in the timing of law passage dates, I find that the average add-on gun law results in a roughly 5 percent decline in gun robberies within the first 3 years. This result is robust to a number of specification tests and does not appear to be associated with large spillovers to other types of crime. (JEL K14, K42)