Measure that bore on the 10 gauge barrel as they can vary. It will make the difference between buying the correct sized mold and the wrong one. A 10 gauge is nominally .775" in diameter. That would call for using a .760 to 765" ball at the widest. You need to leave room for patching, and you want to pick a patching material that is about .005 " oversized for the space allowed. On a rifle, you go thicker, but not with a smoothbore. If you use a .760 ball in a .775" bore, that leaves you .015 " space between the bore and the ball. If you have a .018- .020 patch, lubed, you should be able to load it firmly but with ease, and it will give you good accuracy. However, if you try to use too thick a patch, it just won't load.