That was a rumour put out by a swedish officer back in the 1890's. On a trip to Norway he had bought a Krag and some ammo. When he tried the norwegian ammo in his swedish Mauser, they were hard to chamber. The swedish military even made investigations and found the difference of norwegian and swedish cartridges to be insignificant, even though a few swedish rifles needed a heavier push on the bolt to chamber the norwegian rounds. The norwegian cases and chambers were made towards the max of the specs, while the swedish chambers were towards the minimum. Both were within the specs of the 6,5x55 cartridge though. This difference was rectified only in 1990 when the scandinavian shooting organizations agreed on the 6,5x55 Skan chamber, which size lies between the swedish and norwegian chambers.
Anyway, I don't think that was the difference Rob was thinking about. Norwegian ammo used corrosive primers until the mid 50's. I don't know about Sweden, but I know that until the 1930's, the non corrosive primers were not considered very good (bad ignition in the cold, short shelf life etc). In 1940 when the germans disarmed norwegian units that surrendered, loads of Krags were stored in huge piles. Probably uncleaned. In the 1950's, when the norwegian army surplussed a large number of Krags, it was probably not the best of the best. I guess that's why so many Krags are found with corroded bores. Here in Norway, you'll find lot's of Krags with OK bores. Worn but not rusty.