Mass shootings are almost by definition outliers

In USA, the measure "number of mass shootings per year" doesn't have real outliers, it has a reasonable number every year. In Norway, it's different, you get a streak of zeroes, followed by a 1.

Norway has __considerably__ lower population than the USA. There has to be normalization or your comparison is just bogus.

The comparison is bogus, because you can make the numbers arbitrarily high by subdividing into tiny areas and then conveniently ignoring all the places where they are zero.

Why not subdivide further and claim that Buskerud is a veritable warzone, compared to the whole of USA on average?

Sure, but that doesn't solve the original problem which is that you picked small population countries with number of shootings too low to do meaningful statistical analysis your way because any rare event will have an outsized effect.

There are however plenty of European countries with populations large enough to avoid this problem (Germany, France, Italy, UK...).

So why not compare the US to the EU or parts of it?