Except mass shootings are much more prevalent now than they were in the 1970s. If you see the dependent variable going up as your proposed independent variable is going down, that raises questions about your theory of causation.

Also, America’s current rate of household gun ownership is about the same as Canada, Norway, and Switzerland in the 1990s: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1485564/pdf/cma... (p. 1723). France and Finland were in the low -20s, not exactly disarmed. But our current homicide rate (65 per million) is more than double Canada’s, five times higher than Norway’s, and six times higher than Switzerland’s rates when those countries had similar percentages of households with guns as america does now.

There’s a big difference between gun ownership in those countries vs the US. In those countries, you can’t just toss loaded guns in your glove box (with the safeties off!) like I’ve seen some of my American friends do. Americans have literally zero training on how to use the weapons they own.