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well, that's new to me...I think that it was in American Handgunner where someone reported on a kB! in a light loaded .38Spl. It seems that if the small amount of powder winds up being distributed horizontally inside the case, there is too much surface area of the powder that is exposed to the primer flash. The primer ignites a larger than normal amount of powder all at once and creates a mini explosion inside the case instead of a steady burn from the bottom of the case upwards towards the bullet.