My thoughts would be to step down shot size to increase density. Spinning the shot column causes the shot to sling outward, increasing your chance of missing. A rifled barrel, (or choke, for that matter) would be used more if they gave an advantage to a scattergunner.

I would opt for a used 870 barrel of the cheap kind, and shorten it to 18 1/2".

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James L. "Jim" Rhiner