If my groups went from .7 to "all over the paper" from a .010" change in OAL, I certainly would look for another cause. I would think that a lot of experienced reloaders of the .45 ACP here pay a lot of attention to cartridge OAL, I do. I shoot my 1911 out to 25 yards regularly and occasionally try for 50 yard hits (not a lot to brag about there!) so I like my ammo to be as accurate as I can, within reason (I don't swage/form primer pockets, I don't weigh each powder charge, but do weigh a lot of them, I don't weigh each bullet or case. But I do take care to get each cartridge fairly close; all bullets seated to within .003", uniform "crimps", etc.).
Could you share your "definitive proof?"
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