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I had an off-line question about why neck thickness is important. Reason is that the neck must be able to expand enough to allow the bullet to float free as it begins to move. As the pressure builds, the gas first moves into the point where the bullet fits into the neck of the case and expands the neck, then begins pushing the bullet. If the neck can't expand for whatever reason, the pressure builds up beyond normal limits and the result looks like an overloaded cartridge.
At one time, shooters greased the bullet and case necks, thinking this would create less friction; the result was high pressure because the grease on the case necks wouldn't allow proper expansion. The same thing will happen if the neck brass is too thick.
All rifle chambers are cut with reamers. A given caliber has +/- dimensions. When a new reamer is used, the chamber is maximum. As the reamer is used, it is sharpened and made smaller. When it reaches minimum, it is disgarded and a new reamer is used. The same is true of reamers used to make the dies that make cartridge cases. Manufacturers of arms and ammunition keep the specs within bounds so a factory round will always fit a factory rifle, but with reloads (remember the reamer that makes the reloading die?) yet another factor is thrown into the pot. If cases are made from other cases, we have another element and more room for problems.
[This message has been edited by Jim Keenan (edited April 14, 1999).]
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