AH Fear Not!
There is a fellow that a lot of the folks here on the forum have sent barrels with damaged bores to and had them bored out to fresh, clean metal. Doing the same thing as suggested, they wanted a smoothbore and found a used rifle with a bad bore at low cost. SOME did it on their second gun, and SOME did it since they came to a point in life where they didn't have the eyesight to reach out 100 yards, and they wanted a gun for 50 yards for deer and up close shots on small game.
One of our members in a chap in England, and quite the avid hunter, and the laws there make it oh so much easier to use a smoothbore, black powder gun, than anything modern. He recently posted about using a bored out .45 rifle to take some birds.
And yes, the rifled portion is drilled out. For some folks with a rifle and a damaged bore they sometimes pay extra and have the barrel re-rifled. The fellow that does this knows how far he can take the barrel before it becomes unsafe.
LD