Generally speaking, the powders that give
the best ballistic uniformity and the best accuracy are often those that
most nearly fill the case to the base of the bullet. Ideally the reloader
should strive to fill the case from at least 90%, to perhaps a slight compression
of the powder-about 110%. This insures that the powder positioning in the
case will not cause substantial changes in ballistics, as a result of ignition
characteristics.
An example of less than ideal ballistic uniformity would be a cartridge case
that was only half filled with powder. When held horizontally the powder
might be spread evenly over the entire bottom half of the case. Ignition
of the primer would then result in the entire top surface of the powder ,
a relatively large area, being ignited at once. If the same load were fired
in a vertical position, the primer flash would work primarily on the much
smaller surface of powder in direct contact with the case base. It is true
that the strength of the primer flash would see ignition carried up into
the powder column, but this characteristic would not equate to the ignition
of the same magnitude as discussed in the first example. Filling the case,
within plus or minus 10% is the ideal choice.
It is not always possible to fill a case based on ballistic considerations.
But when it can be done, this is the best choice for the reloader.
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