Leading looks like something has been smeared in your bore. If your bore is not shiny, but has kind of a dull shine then you have leading. Trust me, you will be able to see it.
As to the OP, GPMann knows what he is talkin about. Alot of it is knowing the limitations of your alloy.
Anything above 2%tin is a waste, from what I have heard.
Harder is NOT always better. Many times a softer alloy will make a better seal in the bore.
If pushing cast boolits hard, it is a good idea to get a mold that has a gas check shank cut in it. This allows for a small copper cup (a gas check) to go on the base of the boolit so it can withstand higher pressure loads.
I shoot a 220 grain gas checked boolit out of my TC Encore 454 casull at over 2200 FPS with no leading and exceptional accuracy.
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