The problem here is that you're assuming there's a correlation between guns per capita and shootings per capita, which is similar to the (potential) fallacy that the OP made: that there's a correlation between gun restriction laws and shootings per capita.
Why is the USA then the extreme outlier then when it comes to the sheer number of shootings also highest concentration of guns anywhere in the developed world?
Correlation does not imply causation. As noted elsewhere in this discussion, there are plenty of other potentially relevant correlations, and as also noted, if you cast the net wider than the ODEC we no longer are an "extreme outlier".
I'm not sure how we've deluded ourselves into thinking that the two are not directly related. As other commenters have mentioned, it isn't big scary guns, but the hundreds of millions of handguns everywhere and easily available in the USA that do most of the damage.
See where I just commented on the fact that our murder rate has halved since the 1970-90s period while gun ownership rates have not changed, strongly suggesting we should look elsewhere, perhaps, as I note, demographics: https://news.ycombinator.com/item?id=9838517
Russia's murder rate is twice that of the United States yet their gun laws are far more restrictive. Mexico's murder rate is also about 4 times higher than the United States even though there's literally only 1 legal gun shop in the entire country.
I would wager that a lot of violence in the US is caused by the drug war. Mass shootings are more common than they should be but they're a drop in the bucket compared to all of the violence created by the drug black market.
But drug violence has plummeted from its high point. The drop in drug prices (engineered by the CIA?) meant folks no longer want to get killed over $50.
The drop in drug violence could be a couple of different things but if it is correlated with a drop in prices that would suggest that decriminalizing or legalizing would further drop violence. Just like it did with the end of prohibition.
That's the point: a lot of us, including the article's author, are attributing causes to the amount of spree shootings along to the amount of deaths caused by firearms in general to several individual phenomena but there's almost no way of controlling for a myriad of other factors in play, which include very diverse things like culture, political climate and demographics.
