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Old September 5, 2012, 11:06 AM   #28
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Join Date: December 8, 2008
Posts: 102
I drew up a picture to make sure I had it squared away myself. The Hornady OAL gauge measures the long dimension from the case head to the ogive (or meplat) of the bullet. (The 2.520" dimension in the drawing is only an example for a .308 Winchester cartridge.) However, the critical dimension for the distance between the bullet's ogive and the lands is from the ogive to the shoulder. The shoulder stops the forward movement of the cartridge in the chamber. Therefore, if your headspace dimension (case head to shoulder) is shorter than the modified case, then your bullet will be closer to the lands than you expect.

My guess is that Hornady deliberately uses the minimum headspace dimension for their modified cases. That way you err on the side of having the bullet's ogive farther away from the lands rather than closer/on the lands.

This effectively makes very little difference in service rifles where chambers typically have a long throat. So long you can't seat a bullet long enough to hit the lands and still have the cartridge fit in your magazine. However, it could make a difference in your bolt action rifle or in a rifle with a "match" type chamber.

Please note that I exaggerated a variance in the headspace dimension since .308 Winchester maximum is 1.634" with a variance down to 1.627" per SAAMI drawing.
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File Type: jpg 308 Cartridge.jpg (34.5 KB, 114 views)
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