I chucked it up and make a couple real light passes to get the business end even & flat across the bore.
Once it was all even, the areas that intruded into the bore were a lot clearer.
Then I made a few light cuts inside the edge of the muzzle until the defects were removed, and touched it up with a file, and then polished it with some sandpaper. It ain't perfect and it ain't a pro job, but the muzzle is flat across and the bore is even and concentric.
If you don't have a lathe,there are other ways to get after such things.
You can chuck the barrel in a drill press and use the head of a round head screw of sufficient diameter as a cutter for the inside of the muzzle. Some light work with a file will make the outside less objectionable. You might have to find a fairly hard screw, and you may have to get creative with a file to make it do what you want it to
Or you can go the other way around and secure the barrel in an appropriately padded vise, and chuck the screw head in a drill and just proceed carefully.
Keep in mind, these are strictly down-home bubbafications, and are not precise. If you eyeball it and don't go nuts with the tools, you will come to a point where the problem is either solved, or it is a hopeless wreck that cannot be made right.
You have to work with the problem you have, and understand that the solution is more of a salvage operation; the idea bein' that the thing was un-useable to begin with.