You are correct, sabots are for rifled barrels, fosters for smoothbores. Regarding chokes, most likely your results would be better with a fixed choke than a screw in choke of the same constriction. As a rule of thumb, the less choke the better for accuracy, but for safety, one could probably use up to a standard full choke and be ok. As those were around when fosters were designed, and they're designed to flex for standard chokes. The newer xtra full and XX full postdate foster slug designs, and I would not risk using them in those chokes.
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