At times, a .010" under ball can be made to shoot, as long as the thickness of patching is increased. Years ago, we've found about .022" to .024" works find with the smaller balls. We then tried .005" under balls and the .020" to .022' denim which was easier to find then. Too, the thicker pocket-drills also worked with the .005" under balls. It worked with the smaller ones, except during hot weather - so around here, you could start off just fine, but 5 shots later as the barrel warmed up, or later in the day with the temp climbing 40 degrees, they'd start to fail. This happened to me at Hefley Creek Rendezvous 4 years ago. The load worked up here in PG through the spring barely above freezing weather, failed badly at Rondy with temps in the 80's and higher. (I didn't shoot all summer - silly me). So, I've settled on a .0215" denim patch (10 oz) for .005"-under balls and I use the 8oz. or .020" denim for even larger, bore-size balls. I don't use .010" smaller than bore balls at all, anymore.
Of course, smoothbores require .020" to .030" under balls with appropriate patching.