It's because of the shot hardness factor Lloyd, primarily. The .410 is more subject to shot deformation due to setback because of the proportionately longer length of it's shot column. The longer the shot column the worse the bottommost pellets get "squashed".

The harder the shot, or buffering the shot, lessens the degree of shot deformation and so improves the patterns. Premium loads with harder shot usually equal improved patterns with less fliers outside the useful part of the pattern.

SRH

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