A good question. The first edition of the 'Lyman Shotshell Handbook' has loads using both single piece plastic wads and fiber wads. Unfortunately, even their fiber wad loads use plastic over-powder wads rather than the older hard fiber over-powder wads. So, from their data it is hard to come up with any sort of general rule. I do know that the old Alcan Flite-Max wads raise pressures and so do the very hard one piece steel shot wads. I believe ( could be wrong) that the card over-powder wads do not seal as well as the plastic cups thus giving lower velocities, larger shot-shot velocity variation, and lower pressures. I expect that someone in the mid '60s must have done side by side comparisons but I don't ever remember seeing any. Maybe someone here will be able to come up with data??
C.G.B.
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