You have a lot of good information that I am not going to debate about, but I wouldn't say "Absolutely untrue" as I used the modifier "almost". The majority of guns that are used in these killings do come across the border and are pistols and smaller caliber guns.

Do cartels have .50 caliber rifles and fully automatic, yes of course. But cartels aren't arming their enforcers and hit man with those type of weapons. They are hard to conceal and expensive if you lose them, and at this point most teenagers would rather be a Zeta than a cop, so they have an infinite labor pool. The M16 is often worth more than the person holding it to them. They save those for their big intra-cartel war plays and to scare the government back into submission.

If all the Mexican government had to worry about were diverted military weapons there job would be significantly easier.

That's not even getting to the discussion of how easy it is to drop an auto sear onto the back of a glock in minutes or pay someone in Mexico with a machine shop to do something similar with a lot of rifles.