I have heard that under Bill Clinton large numbers of guns that had been used in crimes and the government claimed were destroyed never were.

The stories i get from some former military is that on at least two occasions the CIA was given tons of weapons that were no longer needed for trials and to be destroyed to be given to groups backed by the US government in foreign countries, And i am not talking about Mexico.
These weapons were reported destroyed or in some cases a extra number/s was added to the serial number to
confuse any one trying to trace the weapon.

The story from Marines is even now weapons taken from in Iraq and afghanistan are collected by the US and may now go to resistances groups like the anti gaddafi rebels or anti Syrian government forces.
The US can deny ever having these weapons and other countries can only blame the county that built them.
Many of these weapons were sold to Iran by the Russians.

The CIA has acquired long lists of AK 47s and other weapons sold to Iran by Russia.

If these weapons show up in other countries the US will not get the blame but Russia and Iran will.

This may be what gave the ATF the idea to flood Mexico with guns and blame gun dealers and the public for them.