Since 1993, violent crime rates have dropped markedly while the number of guns in private hands has gone up markedly. There are some caveats here, including the usual cause and effect notation.
For a lot of reasons, I'm leery of anything Kevin Drum says. That said, he does appear to be an independent thinker, and not beholden to political party or ideology even though he's generally a solid lefty.
One pet theory he's been pushing over the many years I've read him is the lead-crime link.
Violent crime was going up consistently, in the 1960s, regardless of any other known external factors, peaking in the early 1990s. Violent crime has gone down consistently, almost regardless of any other known external factors, since the early 90s.
The theory is that leaded gasoline emissions rose and rose through the beginning of the century and then rose sharply after WWII. Starting in the 1970s, we phased out leaded gasoline. Given time lag (~18 years from early childhood exposure to prime crime-committing years), that basically lines up perfectly with the peak and the decline.
A much more detailed post than I can do justice here:
https://www.motherjones.com/kevin-drum/2018/02/an-updated-lead-crime-roundup-for-2018/Essentially, while I think Drum is a left-wing wacko on a lot of things, he appears to have done some really great work putting this together.
You can credit increased gun ownership with the reduced crime, but I'd argue that's confirmation bias for liking guns in the first place--at least for a lot of people. I think there's a much stronger argument that a lot of violent crime was caused by lead exposure.
