Faustling:

Your assumption is the same one made by Fox News, which claimed that that all weapons not submitted for tracing came from non-U.S. sources. They cited no evidence for this, because there is none. Unless the guns were in a crate being unloaded from a ship from Shanghai, there is no way of knowing where they came from without tracing them. Arms of all kinds are sold (for the most part legally) a few steps from Mexican soil, where they are strictly illegal and extremely valuable. Even if the arms were manufactured in China or some other country, there is an excellent chance that they came through the U.S.

If there's an excellent chance that a gun came via the U.S., the Mexican government would have asked us to trace it so that we could catch the smugglers. There's no reason for the Mexican government to hold back, other than the belief that the attempt would be futile.

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That story from 1996 had nothing to do with American gun laws and stores. It concerned high level professional international illegal arms dealers.

My alternative theory, for which I admit I have no documentary evidence, but which is based on many years of experience with the Mexican cops, is that seized weapons not submitted for tracing disappear in unrecorded ways and end up in the hands of private owners.

Others have claimed that untraced guns came from corrupt Mexican cops and soldiers, and that the Mexicans did not ask us to trace the ones they knew reached their criminals by way of U.S. sales to Mexican police and military -- lest it embarass the government. Your experience with Mexican cops is consistent with that theory.

As for the claim that there is no easy way to convert "semi-automatic assault weapons" to full automatic:

So, for those of you who still possess one of these fun assault rifles,
here is how you can have even more fun with it before they come to take
it away. Converting a semi-automatic AK-47 to fully-automatic.
600 rounds per minute... ohhh yes.

Materials Needed:

1 AK-47
1 twist-tie
and lots of ammunition

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Anyone who does a search on the internet can find lots of conversion schemes. I do not recommend any of them.

They themselves admit that it tends not to be reliable, doesn't work with most brands of ammo (including military ammo, which is the only cheap ammo available in large quantities). Nor did it seem particularly easy -- I certainly couldn't understand what they were talking about. Nor do I have any evidence that it work or is practical -- e.g. that it would _stop_ firing when you release the trigger rather than always continuing until the entire magazine had been exhausted.