Dunno if you watched the video, or whatever, but the reason you noticed an improvement in accuracy with neck-sizing was NOT due to neck-sizing only. Instead it was due to the excellent job Lee Collet Dies do of keeping case-necks aligned with the rest of the case--whether it's FL sized or not. The Lee die does this despite variations in case-neck thickness.
But the reason "standard" full-length sizing often results in larger groups is NOT due to FL sizing itself, but the side-effects of using typical inexpensive FL dies--without checking whether the case necks (or seated bullets) are properly aligned with the case body.
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