Actually, both exists.
Many old Swedish 1867 RB, originally 12.7x44R, RF if military, CF when used for civilian shooting, were converted into shotguns.
That's how you find Carl Gustav shotguns, as I'm not sure they ever made original shotguns (being a state arsenal). But I might be totally wrong on that one.
Husqvarna on the other hand made both military rifle under state contract, and original shotguns (and various hunting rifles like the model 33)
Many of these have one of a kind receiver (not modified military ones), with enlarged rolling breech to cope with the 16 ad 12 gauge cartridges.
Some shotguns are rifled, mainly with straight rifling, due to a law that made it mandatory to shoot moose with a rifle, and many hunter did not want tp have to buya a second gun to shoot fowl...
Many Danish miitary RB (in 11.5x56R or similar) were also sporsterised and turned into shotguns, many of them called "seal guns" and used in Greenland on fishing boat to kill seals when they were caught in fishing nets.