I've resized all three, 6.5 Grendel, 7.62x39 & 220 Russian to 6mm ARC. 6.5 Grendel looks to be the easiest load & shoot after resizing without much more processing (trim & chamfer). The 7.62x39 might need some neck turning first. It may depend on the brand of brass you're using. The 220 Russian shoulder (datum line) is shorter than 6mm ARC so you're only resizing the necks. Bullets need to be seated out to where they just barely touch the lands so you get good firing pin impact. Be sure to use starting load data & light bullets when you fire-form the cases.
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