Author Topic: Preparing Cases for Long-Range Accuracy  (Read 3664 times)

John Boy

  • Free Grazer
Preparing Cases for Long-Range Accuracy
« on: January 17, 2011, 08:17:50 PM »
Step 1 ... Applicable Bottle Neck Cases
Make the neck-shoulder junction to head length and outside case dimensions identical for all cases.
Reason: When we turn the necks, the blade will bite into the shoulder the same distance for each case.
Step 2
Trim the cases to the same overall length.
Reason: This is the second step required to cut the necks to the same depth into the shoulder. It also makes the length of the neck gripping the bullet identical for each case.
Step 3
Chamfer the ends of the necks.
Reason: Trimming cases to length leaves a burr on the neck. Chamfering will remove it, allowing the expansion die to enter the neck easily.
Step 4
Expand the necks to a uniform diameter.
Reason: It is important that the expanded neck fits the neck turning mandrel correctly.
Step 5
Turn the case necks.
Reason: A neck that is uniform in thickness helps improve both bullet release tension and case/bullet concentricity.
Step 6
Make both the flash hole and primer pocket uniform.
Reason: This will produce more uniform ignition from case to case.
Step 7
Check for concentricity close to the shoulder and midway along the neck.
Reason: To know that the cases are straight and to later check the straightness of the dies and the chamber.
Step 8
Weigh the cases and separate by lots not greater than one grain in weight difference or about 0.5 percent.
Step 9
Neck-size the cases.
Reason: Adjust the neck tension and length of sizing to produce the best groups, least velocity spread, and least standard deviation.
Step 10
Determine and size to correct head space.
Reason: This step helps ensure maximum case life, consistent ignition, pressure, and accuracy.
Step 11
Remove a doughnut as necessary and square the head of the case.
Reason: In some instances a doughnut will form in the neck regardless of our best efforts. The doughnut must be removed to produce consistent pressure. The head should also be squared for proper chambering.

The Ongoing Task--Maintaining Case Quality After Fire-Forming
After fire-forming cases, run the necks through the neck-turning tool again. Do not reset the tool. When the case is fired, metal will turn plastic under extreme pressure and stress and will flow to conform to the chamber wall. As it does, the neck of the case will again vary slightly in thickness. Turning the case neck a second time after fire-forming will remove those tiny anomalies.

Keeping cases maintained by cleaning the necks and primer pockets after each firing is something many competitive shooters do. If one looks into a case that has been fired several times, one will see a crust of hard powder that will often flake off.
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