Wondering if anyone knows at what points in the bore does the rifling twist rate change. Does it go from the 1 in 19 or so to 1 in 8 or so in one spot immediately or gradually get faster? Approximately how much of the muzzle 1 in 8?

Ultimately my goal is to counter bore the muzzle by about an inch or so and want to make sure I don't take off much if not all of the tightest twist section.
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