I read an article many years ago, don't remember where, the results of a study in which the researchers had determined the minimum foot pounds of energy required to penetrate the skull and/or vertebrae of a wild turkey.
In those days, lead was pretty much the only option and it was determined that number #6 shot only retained the necessary energy required 100% of the time up to 39 yards. I don't recall the figures, but #5's did so at 60 yards plus. That's when I switched over to 5's.
If the assumptions made in those days was correct, then I would assume that 7.5 shot would reduce the distance a substantial amount.
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