The claim that "90% of the guns used by Mexican drug cartels come from the United States" has been thoroughly debunked (I talked about it here, for example). I have also discussed, here and here, various sources of drug cartel weapons that the forcible citizen disarmament lobby would prefer not to talk about.
The "90%" claim has, in fact, been so thoroughly discredited that even those who tried to make political hay of it have been forced to back away from it. Here's Brady Campaign president Paul Helmke:
Rather than argue about percentages, let's focus on the fact that 20,000 trafficked guns from America have ended up at Mexican crime scenes.
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That's today's St. Louis Gun Rights Examiner. I know it looks like I ripped of
Days of Our Trailers--and I
did get some ideas there, but that's what I was planning to write about even before I saw his piece.