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Old October 16, 2011, 08:18 AM   #26
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That graph seems to be pushing things to the extreme. I've not seen a gun with enough freebore to accomodate nearly half an inch of jump. You would have to push the bullet into the case to the point it slips off the bearing surface. I think for most of us we can make the general assumption that pressure will go down if we seat the bullet deeper and most of us play with seating depth within 0.1", not a half an inch.
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Old October 16, 2011, 08:57 AM   #27
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Note that the graph is for a round nose bullet and it in laboratory conditions and not loaded for practical purposes. I don't think it's behavior matches pointed bullet behavior well in the short distances off lands. I've seen at least one pressure plots showing a pointed bullet's peak pressure dropping 20% just going from land contact to 0.030" off the lands.

The graph is just to illustrate the principle that pressure will turn around below the point where the bullet base starts subtracting a large enough percentage of the powder space available, and that can be seen with modern ammo, especially as the bullet gets longer and the case gets smaller. More often we run into the phenomenon in short pistol cases where the percentage of powder space subtracted by deeper seating is much greater. I think one of the old Speer manuals had the example of a 9 mm's pressure doubling when it was seated something like 1/8" deeper, but my memory is foggy on both the source and the exact numbers. In a big case it's harder to detect.
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