You're right, it's weird. But r + n create a retroflex consonant to begin with, [ɳ], and after that, the s sound follows suit, so we get [ɳʂ]. So the whole 'rns' cluster becomes retroflex.
There's a little more about these consonant sounds in Swedish wikipedia here https://sv.wikipedia.org/wiki/Retroflex_konsonant
and the English article is very good, with useful sound clips
The sound ”r” in Swedish combines with /t d n s/ to create so called ”retroflex sounds” which means that the tongue curls backwards, so ”rs” actually pretty much sounds like an English ”sh”. This also happens after another retroflex sound. So in ”barns”, ”r” first combines with ”n” and then ”rn” combines with ”s” and they all get this retroflex pronunciation which we can write [ɳʂ] and sounds something like ”barnsh”.