A second point By stressed was that dramatic powder granule compression, which occurs when a cartridge fires, results in granule fluidization. Individual particles fuse and reconfigure. Within microseconds the myriad granules become a single mass of viscous fluid. As granule surfaces begin to fuse (at least temporarily), interstitial gas pockets become isolated and essentially spherical. For this reason, further ignition results only at exposed surfaces or where granule-to-granule shearing occurs. Intergranule shearing can open up new paths for propellant gases – we will return to this point.
Owing to adsorbed surface chemicals (which affect both physical and chemical granule characteristics) and size and separation of gas pockets, subsequent burning rate still reflects initial granule characteristics. Critically, individual granules retain the progressive burning gradients imposed by the deterrents. Further, these granules are not necessarily inseparable.
Pressure of a few thousand psi (far below peak chamber pressure) dramatically deforms unignited granules, which are generally located in continuous masses. (My testing demonstrated that a relatively mild pressure of 3000-psi compresses typical tubular and ball powders about 10%, which implies significant deformation.) For this reason, those granules escaping initial (primer blast) ignition show little difference in performance, whether stick- or ball-type.
Regardless of any such details, my pertinent points from the original articles stand (to which, please refer), ideal case designs: 1) minimize interior surface area, and 2) maximize granule mixing rate at the shoulder-to-neck transition. By, who believes it is of significant importance, reiterated a third point 3) minimizing distance between primer flash and the remotest granules. By also points out that we have to add a new dimension to this theorem 4) maximizing primer-blast related heating of those granules in the bore-diameter column, directly behind the bullet – which contains powder that can follow the bullet into the bore as an unignited plug. (Again, refer to the aforementioned articles for background.) See sketches 1a & 1b.
In the following text, we assume that chambering characteristics provide for the base of the bullet shank to precisely align with the neck-to-shoulder juncture of the case.
1) Minimizing
Interior Case Surface Area
This aspect is critical because it minimizes
conversion of primer blast heat into case heat, and for several other
reasons. If this were the only consideration, the answer would be
quite simple – design a spherical combustion chamber. This is certainly
feasible but it will require a new case, drawn with new tooling. Meanwhile,
since that basic shape cannot be achieved with a conventional cartridge,
we will consider conventional (cylindrical) designs. Here, in the
ideal design, powder column diameter equals length.
However, unless someone wants to dramatically shorten a 50 BMG or neck a similarly shortened 416 Rigby to 17-caliber (have fun!), we are in no danger of designing a full-power case that is shorter than it is wide. Therefore, we can say that we should use the fattest case feasible.
(It is noteworthy that progressively fatter cases require progressively fatter actions and that such designs impose increasing levels of axial stress and primer-blast-derived shock – and vibration – into the barrel, characteristics that are probably detrimental but which we will not address further here.)
2) Maximizing Granule Mixing Rate at Shoulder-to-Neck Juncture
Consider
granules trapped behind the case shoulder and not directly ignited
by the primer. Chamber pressure rapidly compresses this mass and thereby
eliminates permeability