The man, who was sitting on the toilet when the bullet struck, was flown by helicopter to a hospital in nearby Ullevål, south east of Oslo.
"It was a hunting rifle with powerful ammunition, and the man was shot in the abdomen, so the injury is serious," Paul Horne, at the local police, told VG.
He said the hunter, who has been brought in for questioning, had loosed off the round just 100 metres from the cabin, aiming to hit an elk he had spotted, and presumably not realising there was a house behind it.
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"The suspect is completely broken and desperate. It isn't an easy situation," Horne added.