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Old February 17, 2010, 05:04 PM   #20
Unclenick
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Join Date: March 4, 2005
Location: Ohio
Posts: 21,017
It's applicable because of exactly what you observed: a load that's compressed in some cases may not be in others. The earlier advice to drop the charge half a grain may or may not change a compressed load to an uncompressed one. There's too much difference in powder densities as will as in internal case volumes, not to mention charge differences or that some powders can compress to higher percentages than others before bulging the case. Too many variables for a fixed number to work universally. Even for uncompressed loads, the internal volume difference in the cases may change the pressure.

All these factors add up to why the standard safety practice is to drop the powder charge 10% and work the load back up, watching for pressure signs, whenever you change anything. Changing anything includes just changing lot numbers of any of your components.

As to the reason case volumes can vary, it is simply that SAAMI and NATO only regulate exterior dimensions of the cases, not the interior dimensions. It is then up to the maker to provide internal dimensions that withstand his intended loads or, in the case of NATO, loads that meet the NATO spec requirements, such as they are.

I too have noticed that the tendency of military cases to be heavier in 7.62x51 NATO did not carry over consistently to 5.56x45 NATO. Again, you have to check the lot of brass you are working with? Assuming external dimensions match (and they don't always; rim thickness and rim and head diameters have tolerances, too), since cartridge brass is 8.53 times more dense than water, you can figure you gain roughly a grain of water capacity for each 8.53 grains lighter that a case is. However, you need a base measure of capacity to know what percentage change that is?

Weigh some cases fired in your gun that you haven't yet decapped or sized. Pick one that's average. This case will be near the size of your chamber, and its internal capacity, thus expanded, is the space the powder burned in at start pressure, and the value that affects peak pressure in cartridges running over about 30,000 psi. Measure the lengths of the cases to be sure yours is also about average length. Fill that case with water level with the mouth (no meniscus) and weigh it again. The difference in the two weights is the case water capacity in grains. Take that capacity and add or subtract the weight difference in another brand or lot of case of the same length divided by 8.53. Divide the first case water weight capacity by the newly arrived at case water capacity and take the square root of the result. Multiply the result times the powder charge used in the original case, and you will be very close to the charge that produces the same pressure in the second brand or lot of case (the one that weighs differently).
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