19-20mm will be fine but just stamp or spin thin cards from braekfast box's and such. Three or four over the powder will be more than enough and pass the chokes easy on loading.
If it gets crusty in the barrels add some grease, bee's wax and olive oil. Or candle wax and olive oil. Then you can keep shooting easy for a few more shots.
If it gets crusty in the barrels add some grease, bee's wax and olive oil. Or candle wax and olive oil. Then you can keep shooting easy for a few more shots.