what size round balls for this?
I don't have the Kibler but I have two other 20 gauge smoothbores with barrels from different makers. I have good results with either .595 balls patched or .610 balls with wadding or wads and cards. By good results, I am looking for sub 4 inch groups off the bench at 50 yards, preferably closer to 3 inch.
I used .600 ball when I got my 1st one because that's what the previous owner recommended, then I got brave enough to experiment with what others have written and found better results with combinations easier to load.