It happens. I doubt a dirty barrel changes a pattern much, but, felt wads compared to the plastic version of same sure the hell does. I remember Zutz giving recipes in books he wrote for loads to tighten or loosen patterns, but, I’ve only played with factory ammunition, and the promo stuff, in 12 gauge, all seems to pattern more densely then my chokes seem to be marked. My variety on the little gauges isn’t varied enough to make the call.

Remember, we call it “hunting” and not “killing”.

Best,
Ted