It’s a good question and I don’t know the answer.

This was interesting:

https://en.wikipedia.org/wiki/Spring_steel

Whether that applies to airgun mainsprings, I don’t know. But it may. Of the ones I know in the list, they are widely available, easily worked and inexpensive, which is what I’d want if I was an airgun manufacturer.

1095 and 5160 are, among other uses, good, traditional, knife steels, and 1070, 1075 and 1080 are often used in machetes and axes. In all cases because they can take a good edge (when properly heat-treated), but are flexible rather than brittle, which is usually the case with harder, more wear-resistant steels.

My impression is that “Swedish steel” means about as much as “Swiss watch”, the latter ranging from a generic quartz movement to a Patek Philippe. The Swedes do make some of the finest steel in the world, but they also make a lot of more ordinary stuff.