There is evidence (old advertisements in particular) that seem to indicate that a lot of rifles were bored out when they became worn and/or rusted beyond use. I suspect that some of the old time guys, like some today, were not necessarily "gun guys", and maintenance suffered, thus accelerating the process.
Still, there are some guns that seem to have been built that way from the start. My own limited smoothbore experience surprised me at the accuracy that can be delivered by a smoothbore at close range (30 to 40 yards). They don't stack the bullets into one hole like a rifle can, but it certainly can supply an acceptable hunting accuracy. We've all heard the story about the British muskets that were not accurate enough to hit a man @ 50 yards. I wonder if maybe that story was exaggerated a bit to encourage Americans, perhaps the other side of the coin that all American riflemen could hit a target the size of an orange at 200 yards.