I do not shoot competitively, but I love to test things, gather data, and learn. What I have found from my testing - and I do have the TRobb sizer - is that resizing larger pellets down does not yield the same results as sorting for and shooting pellets of the same head diameter. I simply don't bother with it anymore, but I do on occasion sort for head size and weight and then use the culled pellets as plinking or tuning pellets. Of course this is for pellets, not for slugs . . .
Of course, for anything to occur in reality, there must be a cause. Here is what I think the cause is, at least at the summary level - variation in ballistic coefficient driven by the different properties of a pellet sized down, either by a sizer or by the barrel. After all, the edge of the head of pellet is very thin, so no matter how it is sized the lead that is displaced simply gets pushed back behind the head. This leads to a different shape than a pellet that does not have much lead displaced. And since the bearing area on the bore is so small, the difference in drag during the shot is not much different - unlike with a slug that has a much longer area to interface with the barrel.
I'll be playing around with this some with testing with my new-to-me LabRadar as time goes on, but it will be a slow process. But either way, my testing to date did not show the same value in resizing as in sorting.
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