They find it hard to believe NRA members would support assault rifles being smuggled to Mexico. Except this is all a manufactured issue. And even if it wasn’t, guns are already illegal in Mexico. The violence happens there, not here where they are largely legal (though not assault rifles). It’s their problem, not ours. You’re not going to disarm criminals who traffic in contraband.
Don’t want contraband smuggled into your country? Work with us to secure the border. It’s quite simple, really.