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I've been dropping whitetails since 1985 (when I bought it) with my 20ga Ithaca Model 37 Deluxe, w/o a problem.
I've only had to shoot @ 100yds three times over those years, but think it the limit for Foster Slugs.
I would only be confidant at 100yds, or a bit more, with a Sabot slug in a rifled barrel, with optics.
Remember that the Ithaca deerslayer barrels were underbored/undersize, to get optimim accuracy with Foster-type slugs, and IME other 20ga smoothbores are much less accurate.
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