I'll take a crack at explaining modern shells!
I believe the rule of thumb for steel shot it that a gun will shoot one choke denomination tighter with steel than lead. For example a Mod barrel will pattern about full with steel. Steel is harder so the shot doesn't deform much or at all on its way down the barrel, nice round shot doesn't stray as much...tighter pattern.
Modern lead alloys are harder, better production makes shot rounder, plastic wads keep the shot from getting mauled up on its way down the barrel. Like steel shot, the shot doesn't deform as much and the result is a slightly tighter pattern. Eigh?
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