One theory I have heard is that when using slow or hard-to-ignite powders, the primer spark itself can propel the bullet forward lodging it in the rifling before the powder actually ignites, which makes the lodged bullet act like a bore obstruction. It would seem a lighter bullet would be more prone to this. It would also seem that low bullet pull would contribute as well. This is why I carefully brush-out any dry lube from case necks before seating new bullets. As a rule, I also do not use light-for-caliber bullets or light charges of slow powders.
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