I had the same thoughts as lancaster when first seeing this post, but held my tongue to see what someone with double rifles would say.
I have successfully used patched, undersized balls in a couple SxS smoothbores - a 12 (.729") and a 13 bore (.705").
I also had very good luck in the 12 bore, using cut-off plastic shot-cup bases form trap wads, cup-up under the the ball and cup-down on the powder,
with spacer wads between them when needed for smokeless loads and without the spacers when using BP loads.
Both worked well, however the patched balls, of course, shot much more cleanly due to the Neetsfoot Oil on the patches, which softened the powder
fouling of successive shots.