Hadn't checked in here for a few weeks. Ironic that the first unread post was after the Boulder mass shooting and late last night there was one in Indiana at a Fedex facility. I don't know what the answers are, I don't thing there is any one singular answer though.
I've always been fascinated by the ATF firearms trace data alter sitting through a presentation about a decade ago. Until recently, the ATF had to search through paper records to determine the origin of the firearm, who sold/purchased it, and perhaps what happened afterward. That is ridiculous in the modern age given the technology that exists. It was my understanding that the gun lobby fought hard to keep everything paper because it was cumbersome and slowed things down.
Now the ATF has something called eTrace which is allowing them to complete tracing much more quickly for weapons recovered in conjunction with crime. Every year they release a report for each state and you can tell a lot about a state from how the data stacks up.
https://www.atf.gov/resource-center/firearms-trace-data-2019
Let's take NJ as an example. Firearms are difficult to purchase here, it takes a lot of effort. In 2019, there were 4,309 trace requests submitted to ATF and they located the state of origin in 2,700 traces. NJ was the source of 19.5% of the firearms, PA was 15.7%, GA 10.1%, VA 9.5%, NC 8.8%, SC 7%, FL 5.8%. In the Northeast PA/GA/VA/NC/SC/FL are often referred to as the "Iron Pipeline" because as you can see 56.9% of the firearms recovered in NJ were from those states. Time to crime in NJ firearms is 12.38 years as compared to the national average of 8.29 years.
Using NC as a comparison there were 16,830 trace requests submitted to the ATF and 13,149 traces were completed. NC was the source of 74.7% of the firearms and the next highest states of origin were SC 6.3%, VA 3.7%, GA 2.3%, and FL 2%. Time to crime in NC was 8.11 years.
The population of NJ is about 9 million and NC about 10 million so it's somewhat of a fair comparison population wise. I find it interesting that there were 4x the amount of trace requests in NC as compared to NJ. Also NJ did not even come up as a top 15 origin source for NC the way NC does for NJ.
We aren't paying enough attention to what happens to firearms after the initial purchase from a licensed dealer.