Resizing - Case Dimension Changes
by: Germán A. Salazar
Some of the noticeable effects of improper resizing are:
- hard bolt closing,
- a click at the top of the bolt opening stroke with normal pressure loads,
- early case head separation,
- damage to the bolt locking lugs,
- frustration leading to assorted venial sins.
Case and Chamber Compatibility
Below are the standard drawings for the .30-06 cartridge and chamber. Compare the dimensions and tolerances of a few key points and you'll begin to develop a feel for the clearances specified.
A case that was reloaded to normal pressure level can exhibit many of the same symptoms of an over-pressure case if it was not properly resized. Just as the over-pressure case creates an interference fit in the chamber, the improperly resized case does not have enough room to expand as the pressure builds and will cause the chamber itself to spring outward slightly, then trap the case as it comes back to normal. The cartridge case needs room to expand in order to then spring back enough to extract normally.
The four key dimensions that must be resized properly in order to ensure proper chambering and extraction are:
Case length, standardized by SAAMI at 2.494" -0.020"for the .30-06.
The next illustration is a print of the chamber reamer used to cut my rifle's chamber. You can see that the shoulder dimension is specified at 0.340" and the base at 0.474". Given the specified body taper of 0.16237" per 20 inches, the chamber dimension at the point we measured (0.390" from the base) should be 0.471".