Guns are absolutely stolen very often. Between 2005-2010, an average of 232,000 per year from burgulary. That is over 1.4 million guns stolen.
https://www.bjs.gov/content/pub/press/fshbopc0510pr.cfm
Those numbers have increased according to everything that I have read recently. I read an article that said it is not between 300,000-600,000 stolen guns per year.
How would I control the flow of guns? Allow law abiding citizens who are willing to pass a licensing test to purchase guns. Continue confiscating and destroying illegal guns, and increase prison sentencing for arms dealers and for possession. Many of the illegal guns in Chicago are supplied by gun shop owners who know exactly where the guns are going.
The cops confiscate enormous amounts of illegal guns. That would absolutely put a dent in the supply if we would stop haphazardly flooding the market with guns that end up being sold or stolen.