If you can find "Scotch Brite" material, a patch of this over a jag, lubed with a fine oil and run up and down the bore a dozen times, will smooth it up considerably.

Once you get the right size balls, I'm sure the shooting will improve. Seems to want to shoot.

I use 15 bore balls (.677") with hard alloys, or .682" pure lead balls in my 14 bore (.69) rifle. It shoots very well indeed.