For those occasions when you want high single-shot damage, but the .50 BMG seems like wasteful overkill.
A bolt-action rifle chambered for .338 Lapua Magnum would seem about right.
edit: Or maybe .375 H&H Magnum if you feel finding .338 Lapua in Sweden in 1989 stretches credulity.
Adjusting the Kotenok, S21 and the Algstudsare to have more damage per shot would do the job.
S21 and Kotenok are both semi-auto, which could create a balancing issue if their damage is too high. Also, it would be a bit strange if they were noticably more powerful than the 7.62mm assault rifles, considering they use the same rounds.
As for the Algstudsare, it’s calibre is simply too small to justify a major buff to its power.
You like to complicate things right?
Fine then:
Any of these could do the trick then, all from pre 1990
The L96A1 Arctic Warfare is actually the current sniper rifle in use by Swedish Armed Forces.
It would make sense for it to be ingame 
Have you ever seen a .270 round in real ? Maybe it’s calibre is “only” about 7 mm but the cartridge case is much longer than the one of .308/7,62x51… And ingame it’s right now more powerful than the 7,62 mm assault rifle ammo.
Aand we’re speaking of a game so a little bit of tweaking here and there is allowed imho
L96 was used in the army in Sweden since 88. I would like to see it in the game.