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Old April 27, 2013, 10:00 PM   #1
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Shape of inside base of rifle cartridges differ by mfg?

I just hand de-primed some 300 Win Mag cartridges; approx 30 each by Lapua, Federal and Nosler. I used Harvey's Deprimer which requires each cartridge be slid over a depriming rod and then "wiggled" until the pin finds the flash hole.

In this very unscientific study, I found:

1. The Noslers' flash holes found the pin almost immediately, as if the shape of the inside of the cartridge base was very concave toward the hole,

2. The Federals found the hole without too much trouble, as if the shape of the inside of the cartridge base was a little concave toward the hole, and

3. The Lapuas found the hole with great difficultly, as if the shape of the inside of the cartridge base was flat (or even concave away from the hole).

Thirty of each should be a big enough sample to rule out a random finding. I did not measure the three flash hole sizes but they all have about the same "slop" with the depriming pin inserted.

It's not a big deal, but could the shapes be different and could the different manufacturers each choose a different shape for some engineering reason?
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Old April 28, 2013, 09:00 AM   #2
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You are precisely correct. With some bench rest shooters, how much powder a case will hold is critical. How much powder the case will hold is determined by the shape. Some brands have beefier webs while some have thicker or thinner case walls.
Same dimensions affect black powder silhouette shooters as many prefer WW cases in 45-70 because you can get 3-4 grains more black powder as the walls are thinner.
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Old April 28, 2013, 09:48 AM   #3
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Another factor is how the flash holes are made. If there are minor burrs on the inside, that affects how easily the decapping pin finds the holes.

Some cases, like Winchesters for some cartridges, actually have the area around the flash holes raised on purpose (NOT burrs). Actually, it is probably more like the area away from the flash holes is lowered, apparently to increase powder capacity in cases with large base diameters.

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Old April 29, 2013, 06:57 AM   #4
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"start low work up slow"

This is specifically why component choice matters.
"all my ammo is mostly retired factory ammo"
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Old May 1, 2013, 09:48 AM   #5
Join Date: November 5, 2009
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Nosler Brass has a VERY generous champher around the flash hole inside the cases, largest I've ever seen.
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