I hear you go to JAIL in Mexico for a single piece of spent brass in your car. I wouldn't want to find out if that's an Urban Legend or real life...
Do not bring any ammunition or even one empty cartridge into Mexico. ...... Federal prison sentences are handed out quickly, with zero tolerance, with no possibility of release on bail or bond, and with very little legal recourse to the accused for the breaking of this Mexican anti-weapons law.
Note also the U.S. Department of State official Warning
http://mexicanmissions.com/mexico/travel/warning.htm