Maybe the long FCs are just nominally advantageous with lighter loads and shorter shot columns, but more significant with heavier loads and longer shot columns. In the instance of the .410 results Stan mentions, the forcing cones become a greater factor as all loads in .410 have very long shot columns. Like the gradual tapers in tight fixed choke. The more shot crowded into the the transition area, the more need to make the transition gradual.
Case in point, I had the FCs lengthened on a Win. 101 thinking I could get close to full choke performance from the modified choked barrel. There was no discernible difference after the work with trap loads. However, with tricked up hunting loads (plated shot, buffer, slower burn rate powders) the patterns percentages were higher with the longer cones. The full choked barrel could throw nickle plated #6s in the 90% range, with 70% inside a 20" circle at 40 yards.
