Measuring Case Head Expansion
Q: I've been told that measuring case head expansion in the extractor groove, just ahead of it, or on the belt allows the handloader to determine the pressure of his reloads. I notice from your loading manual that it described how to use this method. What more can you tell me about it?
A: The system is outlined in detail in my loading manual, and is based on the principal that brass flows under high pressure. By carefully miking the solid head section of a case an experienced shooter can determine when brass begins to flow. Since the case forms the gas seal in the chamber and is the weakest link in the rifle/cartridge system, when this seal begins to fail (flow), it is a sign that you've reached a maximum load. However, I should emphasize that this system measures only the amount of case head expansion; it does not
measure chamber pressure. Pressure can only be measured with a device designed specifically for the purpose. The problem in determining pressure with the head expansion system is that cartridge brass varies in hardness which is an advantage on one hand, because it is a pretty good system for determining a maximum load for a specific cartridge/rifle system. But this does not relate to another lot of brass or another firearm. The best advice is to never exceed the maximum loads listed in your loading manual, because there's a chance these loads may be slightly over-maximum in your particular rifle. They are not likely to be dangerous, but the case head expansion system will reveal this.