Remember, once you cross the border with a firearm or ammunition it is too late! Ignorance of this law will not get you leniency from the police. You will be arrested and sent to jail. Also, the Mexican judicial system is governed by Napoleonic Law which states that you are presumed guilty and must prove your innocence, the opposite of the U.S. laws.
The American consulate in Tijuana, Mexico also posts a similar warning on their official website (emphasis theirs):
Mexico's civil law system is derived primarily from Roman law and the Napoleonic Code and focuses more on the text of actual laws than on prior court decisions. [...] For an accused person, one of the most critical differences is that under Mexican criminal law, the accused is essentially considered guilty until proven innocent.
Is it true that one is guilty until proven innocent in Mexico? Does this have anything to do with Napoleonic law?