How do you know he got the rifle in America and didn't bring it over the border with him?
To be clear, many do get illegal weapons in the US, and that IS a problem - I agree 100%.
Don't know for sure but stats show its easier to get a gun in The US than in Mexico which is why the cartels smuggle guns from The US to Mexico and not vice versa.If he had that gun in Mexico he could have sold it there and easily bought another in The US and made a profit.Also wouldn't have had to take the risk of gun smuggling charges for smuggling a gun into The US.