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Old September 30, 2013, 09:07 PM   #8
Unclenick
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Join Date: March 4, 2005
Location: Ohio
Posts: 21,024
And that's why I asked about the chambering. If you buy a rimless cartridge that headspaces on the shoulder, it's common for the shoulder datum of the new commercial brass to be a couple thousandths of an inch under minimum chamber size. If the gun is new and the chamber is within the new chamber NO-GO gauge length, the case growth after firing will typically be about two to four thousandths over SAAMI chamber minimum.

But the belted cases, like the .300 Win Mag, are designed to headspace on the belt, so the shoulder can be extra short on a new case to ensure that happens and to make it easier to feed the long case. In firing the belted cartridges, brass at the shoulder just blows forward. It can do that easily and without damage since the part blowing forward is the annealed portion. Thus expanded, if dropped back into the chamber, the case then stops against the chamber shoulder before the belt reaches the end of its recess in the chamber.

Note that no harm accrues to the brass from the practice of letting the shoulder blow forward. This is hardly surprising when you consider how many wildcat cases are fireformed to blow their shoulders out and/or forward. All the Ackley Improved designs fire their parent cartridges directly without the intermediates step of using a light charge to form the cases gently, and they do just fine.

Match shooters who use the .300 Win Mag for 1000 yard shooting often prefer to have it headspace on the shoulder rather than the belt. This is to get the self-centering effect of the case and chamber shoulder tapers mating as the firing pin and rearward thrust of the primer drives the cartridge forward during the firing cycle. These folks will adjust their seating dies to set the case shoulder back one or two thousandths from as-fired length, but no more. This ensures the case shoulder makes contact with the chamber shoulder before the belt makes contact with the back of its recess step in the chamber. In the .300 WM, at that point is headspacing on the shoulder instead of the belt.
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Last edited by Unclenick; October 1, 2013 at 06:42 PM. Reason: typo fix
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