That's because it's an impossible question to answer - all guns are "illegal" once they're used for a crime, and it's next to impossible to identify when a gun

That's because it's an impossible question to answer - all guns are "illegal" once they're used for a crime, and it's next to impossible to identify when a gun becomes "illegal" before the crime is committed. However, the Chicago Police Department put out a comprehensive study of guns recovered from crimes between 2013 and 2016 and released the details here : https://www.chicago.gov/content/dam/city/depts/mayor/Press%20Room/Press%20Releases/2017/October/GTR2017.pdf https://www.chicago.gov/content/dam/city/depts/mayor/Press%2... There are some interesting data points in there, like more than half of the 7k guns recovered each year originated out of state and the biggest sources in state are outside county/city that has stricter gun laws. I believe the top three of the stores listed are under indictment or being sued right now. That suggests illegal guns are being sourced from places where it is easier to buy them, which shouldn't be shocking. Another interesting data point is that more than 90% of the guns they traced back to FFL dealers were the only gun they purchased, which suggests that straw purchasers are using new people to buy the guns each time. That goes against any kind of notion that a registry of purchasers would do any good. And more important point is that the "time to crime" was under three years for around 60% of guns they recovered. That means any policy that affects the sourcing of guns will have a pretty short turn around as illegal guns don't seem to have a long street life.