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Loading 10mm vs 40S&W


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If I load 40S&W to an OAL of 1.120, and then load 10mm to an OAL length of 1.180 using the same bullet and the same powder

charge, would you expect there to be much difference in the velocity of the two rounds? Thanks.

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Yes. I loaded 2 different versions of 40 for a while, one at 1.130 (to fit a XDM) and one at 1.180 (to fit my 19/2011s). It took 0.4gr more of Sport Pistol for the longer rounds to make the same 171pf... granted there are differences in the chambers, rifling, etc, but the barrel lengths were similar (5.25 vs 5in).

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Yes the 10mm will be slower assuming the same charge.  Reason is simple, same charge in larger case volume equals to lower pressure and hence lower velocity.

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I made up some 40S&W minor loads. 180 grain blue bullets with 3.4 grains of Vihtavuori N320 loaded to a case length of 1.120." Chronograph result of 10 shots was

an average speed of 762 fps with a power factor of 137,160. Then I tried 10 rounds with the same VV N320 powder weight and same blue bullets loaded to 1.180." The result of the second test was an average velocity of 680 fps with a power factor of 122,400. So quite a difference.

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