Re: Measure this and measure that.
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This is exactly why I thought the larger or smaller dia would change the BS measurement. If you increased the diameter by .001" the ID of the tools would rest farther apart. If the ogive is tangent to the BS that one thou dia change would be maybe a 10 thou (or more) length increase on the BS.
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It's kind of hard for "me" to explain but, when a bullet is placed between the inserts to measure BS length, the ID of the insert that the boattail references off of is not touching the OD of the bullet and is away from it by quite a bit, it's up on the ogive and down on the boattail... not where the bullet is full diameter, or .338" in this case.
The fact that these two reference points are both on angles above and below the BS allows the actual bullet diameter to fluctuate (if it does) and not influence the bearing surface length at all.
I hope that makes sense?