Firearm and Ammunition Penalties:

The Department of State warns U.S. citizens against taking any type of firearm or ammunition into Mexico without prior written authorization from the Mexican authorities. 

Entering Mexico with a firearm or even a single round of ammunition is illegal, even if the firearm or ammunition is taken into Mexico unintentionally.

The Mexican Government strictly enforces its laws restricting the entry of firearms and ammunition along all land borders and at air and sea ports. Violations have resulted in arrests, convictions, and long prison sentences for U.S. citizens, including several who unintentionally crossed the border with firearms or ammunition in their possession. 

U.S. citizens approaching Mexico along the land border who realize they are in possession of unauthorized firearms or ammunition should not try to enter Mexico.

The only way to import firearms and/or ammunition into Mexico legally is to secure a permit in advance from the Mexican Embassy in Washington, D.C. or from a Mexican consulate, even if the firearm is legally registered in the United States.

Vessels entering Mexican waters with firearms or ammunition on board must have a permit previously issued by the Mexican Embassy or a Mexican consulate. Mariners do not avoid prosecution for arms smuggling by declaring their weapons at the port of entry! Before traveling, mariners who have obtained a Mexican firearms permit should contact port officials to receive guidance on the specific procedures used to report and secure weapons and ammunition.