Year or so ago I remember reading in G&A (or similar magazine) about a test where they took a 12ga. with 28" barrel, then they start cutting off one inch of barrel at a time while measuring spread & penetration. At each stage they used various loads. Buck, bird shot, etc. They were surprised that they got below 18" before there was any noticeable difference. I guess part of the inference is that you could go duck hunting with a 20" barrel as well as a 28" barrel. Anyway, wish I could find that article.