OP, are you using lubricated lead bullets? Is there maybe lube in your seating die? Is your measuring method consistent? Maybe lock your caliper to length on one round and see how the others compare.

Once that die is set, it should seat to the same depth every time. Unless there's a buildup of old lube inside the die.

Variance in the bullet length can lead to differences in case volume but it I doubt there's enough to be of concern to most reloaders.

As mentioned above, consistent case lengths are critical to a consistent crimp but that has nothing to do with COL.

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