Simple, you cannot own a gun unless you get it for recreational purposes (hunting). And you need a license. There are two ways in my opinion to slow the flow of guns:

A) Regulate the multiple Gun sales to individuals in the US (or at least make it difficult to buy them close to the border) B) Mexico needs to install better controls restricting what gets into the country.

The gun laws in the United States allow the sale of multiple military-style rifles to American citizens without reporting the sales to the government, and the Mexicans search relatively few cars and trucks going south across their border. So when you combine both factors, then you have a problem. Fixing one without the other won't help.

http://www.nytimes.com/2009/02/26/us/26borders.html?pagewanted=all