I have a friend who had real trouble with his 4 bbl K-80'a 410's, the forceing cones were ROUGH and patterns poor. He spent hours polishing them and it all bore fruit when he broke the intersecting bore/forceing cone junction makeing sure there was no sharp joint. The same guy polished my Briley forceing cones and it was a major improvement on 28, and 20 (I now use .002 choke in both for skeet) The 410 was so much better that it hard to believe, 23 more bb's in my pattern, something like 13% better. I strongly recomend the polish job for 410 tubes and a close check of that area on barrels.
bill
