The hardness of brass is usually measured by Vickers testing. For small thin samples (like cartridges), the micro-Vickers test is used. This involves creating microscopic indentations with a diamond anvil, using a known force. The diagonals of the indents are then measured under a microscope: the longer the diagonals, the softer the brass. This is a very accurate method, and by using caliber-specific jigs, the cases can be reloaded and retested. The Vickers scale of hardness is as follows:
Annealing is part of the manufacturing process of every rifle cartridge case. As brass is worked in the drawing and sizing processes it hardens, and has to be selectively annealed to produce the correct hardnesses at the different parts of the case.
The case head needs to be very hard, while the brass should get progressively softer towards the neck. The neck itself should have a nominal hardness of around 95-120 HV on the Vickers scale.
So how quickly do case necks work-harden with reloading? The answer is: really quickly. Just one pass through a standard resizing die with an expander ball is usually enough to add 20 HV + to the hardness. Seating the bullet adds another 5 – 10 HV. Interestingly the actual shooting of the cartridge has a relatively minor effect. We also found little difference between regular and "hot” loads, but this is just in relation to neck hardness. "Hot” loads are harder on brass for other reasons.
It is apparent that, depending on the die, after only a few reloads, the neck hardness can go from Half Hard to Extra Hard, almost the same hardness as the head. The idea that you only need to anneal your brass every 5 – 10 reloads just isn’t right. To really benefit from annealing, it must be done every reload.
For more detail on brass hardness and the benefits of annealing, see our study Annealing under the microscope.