Norway overhauled their prison system in the late 90s, and their per capita mass shootings since then far outweigh the US.
They're at like 20x the US for 2009-2015. Even if you do the total rehabilitative 20 years or so of data they're at least 4x worse than the US in per capita mass shooting.
A single point of data in Norway makes it the leader? As stated in the article, this is clearly a failure of statistics more than a proof that the US is OK.
America has plenty of mass killings in states with the death penalty.
Results are what matters. Kudos to penal systems that actually understand that instead of kowtowing to public pressure.