Conversely, in the 243, the short case neck cannot cool the plug surface sufficiently to quench burning along the perimeter; hence, as plug passes into barrel, the exterior is burning. This further heats and corrosively damages the steel. If true, occasional variations in how the plug perimeter continues to burn could explain the pressure spikes and general difficulties that ballisticians routinely report with the 243.
(Brass absorbs heat 400 times faster than smokeless powder and several times faster than steel, so cool brass is very effective at delaying ignition and burning of smokeless powder – cold brass can even extinguish contacting granule surfaces that are already beginning to burn!)
Whatever the explanation, longer case necks appear to be useful. I believe that these add to barrel life because such a design allows for a short protective plug of unignited propellant to follow the bullet. If this is true, then not only is a long case neck desirable but we also do not want the case to be too short, else no plug would exist.
Optimum Design
My partner,
By Smalley, and I (Superior ballistics Inc.) have done exhaustive
analysis, both from first principles and in the laboratory, and have
demonstrated that an ideal case design does exist. Such a case has
a powder column (behind case shoulder) near 2.1 times bullet diameter
and uses an elliptical case shoulder. Patented and protected under
the SMc™ moniker, for any given case volume, these parametric design
characteristics provide optimized performance with minimized barrel
wear and barrel heating.