"A lot of benchresters use it on their super expensive barrels, so it's a well proven (and safe) remedy."
That's true. Not only that but they use it from the get go without using solvent first. (I got this from a friend who is heavily into the bench rest game) The only time they go to solvent is to remove the JB from the bore.
One of my rifles had a barrel so rough that it would out in one magazine run. (5 rounds) It would take sevaral hours using Sweet's 7.62 goop to get the ccopper out and then the next time out another 5 round and time to clean again.
I bought cheap firelapping kit and used some very hard cast lead bullets to hold the grit. The instructions said 10 rounds with the rough grit, clean thoroughly, the 10 with the medium grit and so on. because I used cast bullets instead of jacketed, I only did 5 rounds with each grit (there wee 3 grits). The fourth set of rounds was 10 using the JB Bore paste on the cast bullets. I'd cut back to 5 on the grit loads because firelapping will remove more metal at the throat creating a worn thoat as if one had shot 5,000 rounds or so. I decided to be cautious on that subject. I happen to like that particular rifle very much, a Winchester M70 Featherweight in 7x57 Mauser.
Cleaning is now done in the bench rest manner, patches with JB paste, the removal of the paste with Shooter's Choie or some similar solvent.
That rifle still fouls but now i can go through two or three boxes of ammo at a range session if I need to and accuracy is still there.
One your Swede, why not give firelapping a try? Smooth out the rough spots and that should make a difference. I have two milsurp rifles, a 1917 Eddystone in 30-06 and a1912 Styer Mauser that had been converted to 7.62 NATO. Both have two groove barrels that have been damaged by corrosive primers and don't shoot all that well. I'm thinking a good firelapping sure would go a long way toward fixing your fouling problem.
Or, you could put the grits on a tight patch and lap the barrel that way. I've done that a time or two on guns that had rough bores. Just be careful and go slow.
Paul b.