Their penal system probably has something to do with it too. It's the polar opposite of the US in pretty much every way, perhaps even gone too far as it's so friendly that some eastern European people actually want to go to jail because they'll make more money than they do in their home country while also getting free food and a roof over their head so as soon as they're released they'll commit some petty crime to go back to jail. But even with those people, I think Norway still has (one of) the lowest recidivism rate(s) in the world.
And, to be fair, I'll admit that Norway's homicide rate vs that of the US may not be completely linked to the number of firearms. For instance, Norway's robust social safety net probably has a lot to do with them not shooting one another as well.