I don't think there's actually an inconsistency. On page 8 of the book linked, it describes "the charge, consisting of powder and duck-shot ...". Merriam-Webster[1] defines "duck shot" as "a medium-heavy lead shot used in duck hunting". Muskets used smooth (unrifled) barrels until around the middle of the 19th century [2], so they would have been well suited for firing shot shells.
[1] https://books.google.com.sg/books?id=H6F4_9joRkgC&pg=PA7#v=o...