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SEATING DEPTH and CARTRIDGE OVERALL LENGTH (COL)
QUESTION: ANSWER: Please note that the bullets in the illustration below have seven drivebands. The first drive band is cut onto the rear of the ogive and has a different shape to those drive bands that follow. Depending on the cartridge and the length of the shaft, a GSC drive band bullet can have 4 to 7 drive bands. |
QUESTION: ANSWER: Modify the seater plug at C by machining a hole as shown. It ensures that all bullets are seated by pushing the bullet by the ogive and not the tip. Ensure that the seater plug machining is perfectly concentric, otherwise it will still cause runout when the cartridge is checked. |
QUESTION: ANSWER: This does not hold true for GSC drive band bullets in the HV, SP and FN ranges. The good news is that lab testing done at Somchem Ballistic Laboratory in South Africa, showed that loading GS Custom Drive Band HV, SP and FN bullets, in contact with the rifling, or off the rifling, did not make any appreciable difference to speeds or pressures. It is therefore not a problem if load development with HV, SP and FN bullets is started with bullets touching the rifling, if the magazine and action length will allow this. HV, SP and FN bullets are not as sensitive as older designs to speed variation. Tuning for group size is done primarily by varying the overall length of the cartridge. There are two considerations with pressure and COL, if all the reloading components remain the same. When loading with conventional smooth or grooved bullets, pressure increases as the COL becomes longer and the bullet approaches the rifling. At the same time, pressure is decreasing because the bullet is seated further forwards, creating a greater volume within the case. With drive band bullets, the COL pressure component is greatly reduced. The chart below shows the relationship between these factors. |
QUESTION: ANSWER: COL tables are published as a guideline for manufacturers of rifles and cartridges so that rifle magazines are not shorter than the specification and cartridges are not longer. This ensures that all cartridges manufactured will fit all magazines that are manufactured. There is a rule about GSC drive band bullets that is cast in concrete and that rule says: There are no rules! Do the tests at these pages and, if the rifle passes, do the third test below. Test #3: A GSC drive band bullet centers itself in the freebore, To achieve accuracy, it is not required to have ammunition that is as close to the rifling as possible. Do the test whereby you load five or ten rounds and measure the runout of the bullets in the casing. Cycle the reloads through the action of the rifle and measure them again. The ammunition has less runout than what they were before. See the Load Guidelines for HV bullets for more information. |