If the barrel is not to thin you might be able to have screw in chokes installed in the barrel.
It might be less expensive to just buy a new barrel or find a good used one.
As for shotgun barrels having holes in the pattern. I had a beautiful A-5 Browning that would pattern 6 shot as pretty and even as possible, try using 7 1/2 or 8 shot and there was holes in the pattern big enough to throw a ball cap through.
You might want to try a different shot size before you give up on the barrel you now have.