I actually saw this for the first time not too long ago. It manifested itself on my AMP Press. I was seating bullets out of one box of a particular lot, ran out with about 10 to go. I grabbed another box of the
same lot and they all exhibited about 10-15 lb more seating force (shown below in orange).
View attachment 8716008
I kept them all separate, but these were for a trip out to the hills and didn't end up recording the difference in performance. But what I did do was contact AMP and asked for their opinion. They saw that the press engaged about 20 thousandths of an inch earlier, leading them to conclude that the new box had a longer bearing surface, so was engaging the case sooner and staying engaged longer, resulting in the plots shown. They suggested comparing the two with a comparator.
If only I had one of the original bullets to compare...
Fortunately, if you want to call it that, one of my rounds out of the original box failed to fire. So I used that as a baseline and measured it against a number of bullets from the new box. They all landed at .020" longer, give or take.
Again, same bullet, same lot, ordered at exactly the same time.
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