Pete,
In my experience, .410's seem to be especially sensitive to any bore issues that a 12ga may never show signs of. Pitting on one of my .410s causes some weird patterns. A small choke problem caused a really bad pattern also. I also had some problems from a badly lengthened chamber. So, I had to make a special reamer to clean up the chamber and lengthened the cones in the process, mainly just to clean up the mess from the other hack that did the chamber lengthening.