into the powder slightly behind the zone where the first parcel impinged; significantly, this parcel will contain somewhat less energy, compared to the previous parcel. This process continues so that this energy is well distributed into the column of powder behind the bullet – just where it is needed and with surprisingly good distribution. See Sketch 6. Importantly, while this is happening, shock energy transfer into the barrel is spread out in time, which is also beneficial because it should result in milder vibrational excitation.
Additional Thoughts
from By
Extremely special case body and shoulder designs could have
merit. Among the myriad possibilities are: parabolic configurations
(with no distinction between body and shoulder) and elliptical and
hemispherical shoulder designs. While interesting, parabolic and elliptical
designs could tend to focus the reflected primer blast too tightly,
so the hemispheric (or possibly a parabolic) design seems most interesting
to me (see sketch 6).
Powder column width-to-diameter ratio is an important consideration when using a conventional shoulder. By believes the proper mix of the above-discussed characteristics leads to a design where interior diameter is about twice bullet diameter and shoulder angle is near 30 degrees for small capacity applications (shorter cases) and near 40 degrees for large capacity applications (longer cases) – he agrees that the hemispherical design may well be a superior choice. We are both impressed that, through trial and error, Ackley seems to have gotten shoulder angle quite right for hunting cartridges and the target fraternity seems also to have managed quite well – such subsequent proof of empirical results is not unusual. Formerly, it was difficult to find cases with sufficient body diameter to meet By's proposed "twice bullet diameter" criteria. At least for the smaller sporting calibers, this is no longer true. Therefore, new short fat designs should become more and more popular for this and other reasons….
Two other points By felt very strongly about are: 1) bullet mass will influence "ideal" shoulder angle – a least with a conventional shoulder design – and 2) bullet base configuration might also influence ideal shoulder angle. I might add that effective bullet mass, as influenced by bullet specific characteristics (core & jacket materials, etc.), neck tension, crimp, friction proofing and bullet-to-rifling jump, which will all influence how fast the bullet initially accelerates, is also an important consideration
Preliminary Conclusions
If the above
analysis has merit, it seems evident that there is no such thing as
a universally "ideal" conventional shoulder angle. As noted previously,
each powder column length-to-width ratio, each effective bullet mass
and each case volume favors a unique angle; this angle depends upon
which of the above discussed effects dominates in that particular
system. On the other hand, if the Thermos Bottle design works in the
manner this analysis suggests, then that basic design might work extremely
well across a wide range.
Before closing, I must note that I have been told that, early on in PPC experimentation, a design with a large body-to-shoulder radius was tested; that particular combination proved to generate excessive case stretching, owing (I presume) to lack of adequate headspace control. If that is a general characteristic of such a design, I can only hope that cases with a wider shoulder will mitigate the effect sufficiently to be useful. Time will tell….