NPR’s fact-checking  had caveats, such as when Donald Trump railed against Chicago’s strict gun laws, NPR agreed that Trump was right. However, they added that the illegal guns flow in from the neighboring state of Indiana. Trump was correct about the strict gun laws in Chicago and blaming Indiana for guns is not an accurate fact check:
Chicago has been an epicenter of gun violence and according to FBI data, homicides in that city account for a substantial proportion of a rise in violent crime. Scholars say that violence there is largely concentrated in a few neighborhoods. While Chicago does have strong gun regulations on the books, many weapons flow from other places, including Indiana, according to this report  in the Chicago Tribune.