I can't be sure, but I wonder if the early burn rate could effect how the ogive curved surface of the bullet engages with the rifling. If the jump from the cartridge case through the chamber to the throat engagement of the rifling was unstable (the propulsion was not evenly flat against the bullet's back surface) that might do it...
Marc
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 Igitur si vis pacem, para bellum -
- Therefore if you want peace, prepare for war.
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