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08-28-2014, 05:44 PM | #3 | ||||||
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By Tom Armbrust:
To show what happens with various crimp depths, another test was run in the Hodgdon Ballistic Laboratory using one control reload and different crimp depths. Unless otherwise listed, the data in this text was created using a standard depth of 0.055, which is a bit short of 1/16. However, some manufacturing variations exist in which case handloaders are urged to use a factory equivalent crimp depth for that particular shotshell. The test reload was checked for pressure and velocity at 0.020 increments. The test reload was assembled like this: Shell: Winchester 12 GA 2-3/4” AA CF Primer: Winchester 209 Primer Power: 20.0 Grains of Hodgdon Clays Wad: Winchester WAA12L Shot: 7/8 oz of Lead Shot The resulting ballistic data was as follows: Crimp Depth Velocity (fps) Pressure 0.030" 1,308 f/s 9,300 PSI 0.050" 1,329 f/s 10,500 PSI 0.070" 1,351 f/s 11,900 PSI 0.090" 1,363 f/s 13,100 PSI The importance of crimp depth, then, should be obvious: crimp depths to either the high or low side of normal will directly impact pressure/velocity results. |
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The Following 2 Users Say Thank You to Mark Garrett For Your Post: |
08-28-2014, 05:59 PM | #4 | ||||||
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It would be interesting to see if there is that much variation in pressures when using a roll crimp.
Does Tom address that? |
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08-29-2014, 07:27 AM | #5 | ||||||
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This seems to discount the theory whereby a taller shot column - as with the same weight (mass) of bismuth vs. lead - will result in increased pressure due to more surface/friction of the shot against the bore, as the shot is accelerated. Just sayin.
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08-29-2014, 07:49 AM | #6 | ||||||
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How is that ?
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08-29-2014, 08:35 AM | #7 | |||||||
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Quote:
Case in point. Sherman Bell's data for the Short Ten shows a Fed Hull, Win 209 primer, 32gr of SR7625 with SP10 wad and some filler for both 1 1/4 ounce lead and 1 1/4 ounce of Bismuth. The lead load uses a folded crimp and generated 6700 psi. The Bismuth load using a Roll Crimp (which reduces pressure) generated 7100. I suspect if that Bismuth load had used a folded crimp there would be somewhere around 1000 psi greater with the Bismuth. |
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08-29-2014, 08:46 AM | #8 | |||||||
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I believe it would have an effect but compared to the other factors, especially with modern plastic wads, that effect would be negligible. I also believe that all else being equal a slower burning powder will produce less felt recoil than would a faster burning powder. The results of tests of this theory are subjective depending on the individuate test participants. I like theories but am forced to accept hard data. Mark
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Don't hunt with a gun that will embarrass your dog! USMC Retired USMC Distinguished Marksman USMC Distinguished Pistol Shot NRA Benefactor - Ring of Freedom member |
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08-29-2014, 08:46 AM | #9 | ||||||
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Mark G, thanks - the also was true when going with less crimp. I believe Mark O. in a previous post had some info about shot hardness effecting pressures. JMHO - lead shot will flatten a bit in the back part of the shot column, acting like the cushion part of the wad. This allows the expanding powder gas to expand more easily, meaning less pressure. Might be all wet with that theory, but there's some reason hard shot raises pressures. The roll crimp issue. Mark had also commented on deep roll crimps raising pressure and I would agree with him.
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Paul Harm |
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08-29-2014, 08:48 AM | #10 | |||||||
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