  We propose a simple mathematical model to describe the evolution of violent
crimes. For such purpose, we built a model based on ordinary differential
equations that take into account the number of violent crimes and the number of
legal and illegal guns. The dynamics is governed by probabilities, modeling for
example the police action, the risk perception regarding crimes that leads to
increase of ownership of legal guns, and so on. Our analytical and numerical
results show that, in addition to the rise of criminality due to the presence
of illegal guns, the increase of legal guns leads to a fast increase of violent
crimes, suggesting that the access of firearms by civilians is not a good
option regarding the control of crimes.
