Case interior shape (larger diameter cases allow a greater
percentage of primer plume to dissipate laterally; which might penetration
depth) but less primer plume energy is wasted heating case walls (dissecting
any used rifle case will clearly demonstrate where primer gases have
condensed upon interior surface, these residues are black)
Case filling
ratio – volume of uncompressed charge versus volume of propellant
chamber – (the fuller the propellant chamber or the more compressed
the charge, the lower the ermeability of the charge mass, so the lower
the effective penetration depth)
Observational Support
Perhaps
a series of examples from experience will be helpful in understanding
these differences. First, let us consider the 44 Magnum, a cartridge
with a propellant chamber that is about ½-inch long. A typical midrange
load in this case might use about 8 grains of Unique. Experience proves
that, in such a load, primer plume always reaches bullet base – as
evidenced by primer residues on base of recovered bullets. Since this
charge fills only about 60% of propellant chamber, it is unsurprising
that plume can reach bullet base. Conversely, a typical full-power
load in the 44 Magnum using about 25 grains of W296,which fills essentially
100% of propellant chamber, when ignited by the very hot CCI-350 primer,
acts far differently (CCI-350 is hotter than most rifle primers).
In such a load, primer blast never reaches bullet base – this author
has xamined hundreds of recovered bullets fired )