I would not recommend taking a firearm into Mexico, even if there are some legal ways to do it - however for personal defense, machetes are perfectly legal. I don't get that, but most gun laws are by nature pretty stupid and illogical. I had heard that bringing hunting rifles/shotguns was legal if you had valid hunting licenses too but on checking I found no such loophole. What I have not found out about is whether black powder guns are considered to be firearms, according to one source YES according to another they are not, but neither is a Mexican lawyer so I don't know yet.
Still, if I had a good lead on where to find one of the lost missions, I would sure consider a little "vacation" to Baja, with a stop at a hardware store of course!
Here is the 1910 Mexican treasure trove law
This is from 1910 so may not be accurate.Art 238 Treasure Trove
Art 238 Treasure Trove Hidden treasure belongs to him who discovers it on his own property but if found on public property or on that of a third person the finder shall have one half and the owner of the place the other half. If the thing is discovered be of interest to science or the arts it shall be delivered to the Nation at its just price which shall be distributed as above The discovery of treasure must be accidental in order to enjoy the above rights nobody on his own authority may make excavations borings or other work on lands of another to seek for treasure and should any be discovered by such means it shall belong entirely to the owner of the land Whoever without the consent of the owner commits such acts shall pay all damages caused and the cost of restoring things to their orignial condition if he be a tenant he shall forfeit the term although it is unexpired if the owner so requires If the treasure is sought with consent of the owner any agreement as to division shall be carried out in the absence of such agreement the expenses and the treasure discovered shall be equally divided The same rules of division apply between landlord and tenant when the tenant discovers the treasure if discovered by a third person the division is between the finder and the owner If the owner himself finds the treasure on his own land held by a tenant the latter has no part in the find but may recover damages for any interruption of his occupancy or injury to his estate in seeking the treasure even in case no treasure is found By treasure is meant the hidden deposit of money jewels or other precious objects the legitimate source of which is unknown it can never be considered as a product fruto of the estate If found upon land held in emphyteusis the emphyteuta shall be considered as the tenant or usufructuary Mining the cutting of timber pasturage etc arc governed by the special laws on those subjects Arts 759 772
Roy