John Hammar broke that rule and now he’s in a Mexican jail.
The government of Mexico is very strict about this. They want visitors. They want people to come to their country and to spend their money. If you do you can do a lot of things but bringing either firearms or ammunition is not one of them. He and his family are trying to convince everyone that the gun was an antique but this is no antique:
The UK Daily Mail say his parents are upset about his incarceration.
It is possible to bring shotguns into Mexico for hunting purposes. The restrictions are very stringent and they require a permit prior to attempting to bring it in. Hammar did none of these things.
His best chance is to try to get the court to set a fine and have his family pay it. It will be expensive and his gear – including the RV he was traveling in will probably be confiscated. He will be sent out of the country and probably never allowed to return.
If you want to support his family’s efforts to get him back they have set up a ‘Free John Hammar’ page on Facebook.