There are two ways of determining the hardness of brass. One is to microscopically examine the grain structure. The softer the brass, the larger the grains will be. Typically, very soft brass will have grains in the range of .040 - .070 mm. Medium soft will be .020 - .035 mm. Once the brass is mid-range (approx. 90 – 100 HV on the Vickers scale) the grains will be .005 - .015 mm. As the brass gets harder and harder, the difference between grain sizes becomes more difficult to distinguish, meaning this is not a great technique for the upper end of hardnesses relating to case necks. Also, the test process destroys the case, meaning it is impossible to do progressive tests on the same cases.
The second method is called the Vickers test, or for small thin samples (like cartridges), the micro-Vickers test. 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 consistent is factory virgin brass? Without naming brands, we have tested top quality virgin brass with neck hardness as low as 90 HV, and others as high as 150 HV. The difference is in the finishing process. Some brands give a final neck anneal to the brass before packaging. That puts it in the softer range. Others give a final resize. Those are in the harder range. The good news is that we have generally found excellent consistency within each lot of any particular batch we have tested. It is fair to say there is no "right” neck hardness for factory brass. What does matter is consistency.
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.