There is no "iron pipeline" of guns trafficked from low gun control states to high ones.
There is a rust pipeline of legally migrating guns mainly between neighboring states.
Take-aways
Crime guns are largely local
One reality is that most crime guns are retailed in the same state where they were recovered. Across all states, 68% of traced guns were retailed in the same state where they were recovered.
But there are significant variations. Due to decades of restrictions, Washington, DC (not a state, but worth including since it has the highest gun homicide rates in the nation) had only 5% of its traced guns originating there. Little wonder, since for a long time there were no DC gun stores per se. The next lowest is 20% (New Jersey, also highly restrictive) and the highest is 85% (Texas).
This is where things get odd and where politicians launch irrational agitprop. And it gets odd on two vectors: where crime guns come from and how long it takes for those guns to be used in crime.
The short story is that it takes a very long time for a retailed gun to be used in a crime, regardless of whether it was retailed in or outside of a state. If "iron pipelines" existed, you would see a very short interval ("time to crime," or TTC) and it would be consistent among states with strict gun retailing laws.
Neither is true.
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(Gun Facts Source Page)