I find LimaPapas observation most interesting as they mirror my findings at the pattern board almost Identically. I imported some Italian 10 ga O/U's which had .042" choke in each barrel. Base patterns were 64%. Steel No.2s. Starting with the lower bbl I opened them up in .003-.004" increments. A steady increase in density and improved evenness of distribution were noted until at .018" constriction Where it was printing an 84% pattern .Set the top barrel to this dimension and continued with the lower bbl until at .008" restriction i got a very good 60% pattern. I've subsequently used the .018 & .008 on a number of guns , both 10 & 12 ga with very good results. I did balance the chokes to give cone length to choke parallel lengths with a ratio of 1:1.3 to accommodate the longer shot column of steel vs lead , weight for weight.