My barrel is .577 (tested with pin gauge). Is a minie that came .590 even able to see sized car enough down, or should it go to a pile to be melted down once I get casting?
What mold do you use? I've been considering a lyman 575-213OS, I figure the lighter bullet should be a little more easily stabilized in the 1:72 rifling, but you've a great deal more experience than I do.
I don't know about your first question, never personally had to deal with bullets that were over the bore size. I started with the Lee .575 mold and worked up to larger calibers.
My thought is, if you already have the sizer - how's your accuracy? If they don't size down well then melt them. if they do then shoot them.
With my Enfield I had been using the RCBS "skirmisher" mold in .576 for at least 10 years.. Still have the musket, but the lock's sear is badly worn.
But now for the last two years I have been mostly using a Pedersoli Lorenz in .547. COVID1`9 cut into that, stopped the shoots for a while, or i would know more than I do. But I bought all three of their .547 molds; big Minie, small Minie, and expansion bullet, which is an alternate technology.
Off hand I don't see the logic in equating "lighter bullet" to "more easily stabilized." Remember, the 8-inch howitzer shell is more stable than a 105mm shell. And more than 6 times as heavy.
I do see that shorter bullets are more easily stabilized in slower-twist barrels, according to a ballistics formula I have seen. .
But my musket experience says working up a bullet and charge combination that reliably spreads the skirt of the Minie is what keeps them from keyholing. You want a heavy enough charge to spread the skirt, but light enough that it doesn't knock you around while you try to hold the sight on the target. And you want soft lead, not hard lead, so that skirt will expand. And if the skirt of the Minie is too thick, it takes a higher charge to spread that skirt. so take a good look at the bullet, not just its caliber. If possible get friends to give you a variety to try, and then you buy the mold that your musket likes best.