The only way to legally import firearms and ammunition into Mexico is to
secure a permit in advance from the Mexican Embassy in Washington, D.C. or
from a Mexican Consulate in the United States. Mariners who have obtained a
Mexican firearm permit should contact Mexican port officials before
attempting to enter Mexican waters, to learn about specific procedures to
report and secure weapons and ammunition.


http://travel.state.gov/travel/cis_p.../cis_1756.html