I was talking with a guy the other day that has a rifle like mine and he uses a .565 rb with 10 oz canvas (.025) soaked in moose milk. Says the smaller ball is better because of the progressive depth grooves. I have used .570 rb and .015 patch.
The instructions that came with my rifle calls for a minie of .575 dia, 50-70g of ffg or fg and I have read several places where people say you should use a minie .002 under bore size.
David Minshall, you, and others recommend .001 under bore size.
I have tried both and have yet to notice a difference, maybe it's because I only shoot 50 yds.
I was shooting 65 and 70g of ffg and 60g fffg using .5758 530g pritchett, and .575 and .5765 566g shallow base P/H
Next time out I'm going to try less powder maybe 50g ffg and 45g fffg.
Ordered some 460g 575213 OS minies to try also, even though I've read these rifles do better with a heavier minie.
Most of my shots are 3" high which is understandable at 50 yds, but I'm also right on most shots, even when I shot the rb.
I think I may have a flinching problem, or the trigger pull is to heavy.
I know I don't have a problem with my 50 cal. T/C Triumph at 100 yds, but it has a scope.
I doubt you'll need any sizing, they should come right from the mold. Bullet sizing is often necessary for modern firearms but it is never desirable, sizing never can improve a cast bullet, it can't make a bullet better than it dropped from the mold, only smaller than straight from the mold. The most accurate minie is one which fits the individual bore snuggly, not so tight as to be hard to ram down but not so loose as to fall down the clean bore. Many serious competitors buy a custom mold to cast a bullet slightly larger than the bore and then size it down a tiny bit to a perfect fit. They may be using that one mold for several rifles with slightly different bore specs and have one custom made sizer for each rifle. But if you are buying pre-cast minies or casting from a mass produced mold I think they are much more likely to be too small than too large.
In other words, don't assume you'll need a sizer until you find you actually do need one and then, if you need one it will probably need to be custom made.