Gun registration leads to gun confiscation. It happened in Germany, Cuba, China, and New Orleans.
One of the limits to our government's ability to set up a gun registration database is that the BATFE must purge their records after 90 days.
However there is no such limit on the United States of Mexico.
Just like it is illegal for the CIA to monitor the email traffic of US citizens from US soil, it is perfectly acceptable to monitor those same emails from a foreign country, say Australia.
Now if the "temporary" ATF firearm purchase records are "shared" with Mexico it would allow a permanent database to be built and stored on Mexican soil. This data would then be "shared" back to the US to help out the gun confiscation efforts as needed.
Gun confiscations in NOLA
You can Google your own sources for Nazi/Communist gun control policies.