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.
https://worldpopulationreview.com/country-rankings/mass-shoo...