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Old July 7, 2019, 08:35 PM   #12
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Join Date: April 24, 2010
Location: Ohio
Posts: 580
Originally Posted by D F Delozier View Post
The phenomenon of "bumping up" as you call it, is know as obturation and will account for a miniscule amount of expansion on a solid round ball with typical relatively weak revolver charges, it's the principal upon which the minie ball works, but they have that thin hollow skirt on thier base, the round ball doesn't. The expansion you're finding is most likely caused by impact with your stopping medium. That's why the forcing cone is there, to guide and squeeze the ball to fill the bore completely, and it needs a larger than groove diameter ball to do it's job. This wouldn't be the first thing an "old timer" was wrong about. This has lots of benefits, better accuracy, more efficient use of powder, less leading caused by blow by gases. The only down side is a very slight increase in recoil for a given charge,caused by the better barrel seal.
Bump up or obdurate same thing. The forcing cone is there to correct slight error in cylinder alignment mostly but not to squish down over groove diameter bullets. The impact medium I was using was water I doubt that would have had much effect on obduration in any case the lands and grooves were clearly present on the periphery of the ball. Recall that I said I reamed all of my guns to groove diameter for a number of years. Gave it up because I noticed no improvement in accuracy. Sometimes "old guys" know stuff.
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