What are they dropping at now?
I shoot "as cast" bullets that are .311" through a .308" bore without issue. I would just set the gas check and then load them at whatever size they come out at.
If you are gas checking them I would load them using data for jacketed bullets.
Cast bullets are a lot more forgiving than jacketed. They will squeeze down to the bore easily without any pressure problems. I would say to start off at the minimum load then work up until you find an accurate charge.
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