First muzzleloader was a acquired in the early 1970s. This was after shooting a number of muzzleloaders previously. Many were originals. My ‘mentor’, who owed and shot multiple originals thought WD40 was the way to go for bore protection. Since then, the only rust I have seen/experienced involved muzzleloaders not treated with WD40. Might be the an exception, but I am sold. Lived in North Carolina since early 90s. No rust problems in more muzzleloaders bores than I can count.
For full disclosure, can NOT recommend WD40 for any mechanical device, say for example a lock or bolt from a modern style rifle. The stuff gunks up the mechanical device. A white frosty residue. Below freezing your are screwed. Just my opinion.