I would go 9.5, the first year I loaded my own shells all I could get were 9.5s, crushed birds without issue, still have a few rounds left. I am shooting a 20 so I went to 9s but in a 410 no question I would go 9.5
The TSS pellets are extremely consistent, my shell calls for 1.625, I literally drop in the last pellet and get 1.626 99% of the time. I think that is an overlooked part of the impressive patterns the shot produces!
