[Wyosmith]

I am a full time gunsmith with over 35 years of experience.

My advice;

Replace the barrel or sell the gun for parts.

I am so sorry, and I wish I had more encouraging words to say but I would not try to repair one that bad.
I would not consider it to be a safe repair.

You can replace the barrel with a new barrel of the correct gauge, but it's a very involved process. You must un-solider the barrels and ribs and remove the old damaged barrel from the monoblock. You then have the new barrel blank machined to the exact contour of the old one. Re- solider the barrels back and re-regulate them. Cut a new chamber and choke in the new one, and then reblue them.
All in all, the repair is probably going to cost at least as much as replacing the gun with one just like it, and it may even cost more then the value of the gun.