I also had "some" luck with my 13 bore straight rifled barrel, using a 16 bore ball, with a 14 ounce denim cloth patch, loaded in brass cases.
The groove dia. in the barrel with the standard 16 bore chamber, was .705" which is 13 bore. The thin brass case allowed loading a true .662" 16 bore 1 ounce round ball, with the denim patching, over 2 1/8" hard cards and 85gr. 2F GOEX.
My shooting with this combination was at 28 yards, from an elbow rest on the bench and the group went 2 1/2", strung out in a line.
Seems to me, long time ago, I posted all of this here, on the site when I had a photobucket account.
I also posted a picture of the 12 bore round ball load COMPONENTS using cut-off 12 bore steel shot wads, as the bottom of the wad was nicely cupped to hold the ball, while the bottom gas seal was stronger than the trap wad gas seal I had been using.