Efficiency: Why Case Shape Matters
M.L. (Mic) McPherson, June 2005
Synopsis: While this has taken more than 150 years, today’s shooters can now begin to realize the benefits of cartridges that are designed for maximum efficiency. These advantages include increased barrel life, superior accuracy, reduced felt recoil, and reduced barrel heating. Because, in hindsight, the reasons that case design matters are easily understood, the question becomes: “Why did this take so long to figure out?”
Let us consider a conventional sporting, centerfire cartridge. Experienced shooters know instinctively that, for any given bore size, a larger case accelerates any given bullet to greater muzzle velocity. Similarly, it is obvious that, all else being equal, loading to increased pressure results in greater muzzle velocity. However, overall cartridge efficiency is hidden in the details of case shape.
Sequence of Events when a Cartridge Fires
What may not be so obvious is what actually happens when a cartridge fires. Here we will review a somewhat simplified version of events that occur after the firing pin strikes the primer.
First, the primer explodes. The pyrotechnic primer pellet burns very rapidly, releasing hot gasses and particles. These still-reacting components stream through the flash hole and into the combustion chamber and thereby ignite propellant granules that happen to be located near the flash hole. Testing has proven that the primer does not directly ignite propellant granules that are more than about one-half inch forward of the flash hole.
Simultaneously, as granules begin to burn, the entire charge is compressed toward the front of the case, behind the case shoulder (if any) and bullet. This compression results from two factors: first, the primer shock wave hammers into the base of the charge and thereby transfers momentum; second, gas generated by inchoate propellant combustion and continuing primer pellet combustion generates gas pressure, which is biased near the flash hole. First, the bullet creates a gas seal at the front of the case. Second, while the charge is initially porous and permeable to gas flow, such flow is retarded because the conduits are tiny and convoluted — with the entire combustion event completing within about 1/500-second; gas pressure simply does not have time to equalize throughout the charge.
Therefore, as burning generates more gas, pressure is progressively biased toward the charge base; hence, the unignited propellant mass progressively compresses, thereby plastically deforming the granules. This is a self-supporting reaction, the greater the compression, the harder it becomes for gas to pass into the charge (conduits become ever tinier and ever more convoluted) so that, very soon, the unignited mass is essentially impervious to further gas infiltration. Thereafter, no significant amount of additional hot gasses can infiltrate into the unignited mass; subsequently, ignition and combustion can only occur at exposed surfaces. Even while gas can penetrate, because the front of the case is essentially perfectly sealed by the bullet the penetrating hot gas cannot carry sufficient heat energy into the unignited charge mass to result in ignition beyond the first quarter-inch, or less, of the mass that was not directly ignited by the primer. Normally, in most rifle loads, no bullet movement has occurred while this is happening.
Cylindrical versus Bottlenecked
Compare the 308 Winchester and the 45-70 Springfield. In each, the primer will ignite about one-third of the total charge. Before compression can seal off the rest of the charge, secondary ignition into the rearward surface of the unignited mass will penetrate sufficiently so that, perhaps, another one-sixth of the initial total will ignite. Hence, about one-half of the charge will be ignited; the remainder will be a more-or-less solid chunk that is burning only on the rearward face.
As chamber pressure continues to build, pressure acting through the unignited propellant chunk eventually becomes sufficient to begin to force the bullet into the bore. To do so, in the 45-70 Springfield, all that is required is that the total force on the bullet base (force on the base of the unignited mass minus friction between that mass and the case wall) exceeds the force required to push the bullet out of the case.
Conversely, in the 308 Winchester, the perimeter of the charge is trapped behind the case shoulder. Therefore, pressure must built up sufficiently to shear a plug through the trapped propellant mass. When this happens, it creates a propellant plug of approximately bullet diameter. Line-of-sight ignition occurs along the perimeter of this plug and along the interior surface of the cylinder of trapped propellant behind the case shoulder. Ignition along these surfaces significantly increases the total area of exposed propellant with the surface burning, compared to the 45-70 (where the entire mass simply accelerates into the bore, while burning only along the rearward face). Therefore, overall ignition rate is significantly increased. Contrary to intuition, this is the main reason bottlenecked cases require the use of slower burning propellants, when compared to cylindrical cases with a propellant chamber of similar length and when shooting bullets of similar length.
Recoil is a Function of Mass and Acceleration Rate
It is demonstrated that, at least initially, in the 45-70 a considerable amount of propellant accelerates into the bore behind the bullet. In a typical loading, this could amount to about 30 grains. Conversely, in the 308, the propellant plug would contain about 10 grains. Equally, energy consumed by accelerating solid propellant cannot contribute to bullet acceleration; hence, case designs that accelerate less solid propellant into the bore accelerate bullets more efficiently.
An understanding of the above will help one realize why case design matters. For example, consider a very fat and very short 30-caliber bottlenecked case. No such case is readily available but we could certainly create a case that held just as much propellant as the 308 Winchester but with a propellant column only about one-half inch long. In such a cartridge, primer ignition will reach the bullet base. In this instance, as pressure becomes sufficient to dislodge the bullet, no solid propellant plug will follow the bullet into the bore. All else being equal, the accelerating mass will be minimized.
This means that the gun will initially accelerate more slowly into the shooter’s shoulder — less felt recoil; less total work will be done on the barrel — less barrel heating and wear; and more work will be done on the bullet — more velocity! Any shooter who has done a side-by-side comparison of otherwise nominally identical guns chambered in 300 Win Mag and 300 WSM, will agree that the shorter case generates less felt recoil, despite essentially identical ballistics. This is explained by the fact that the 300 WSM accelerates far less unburned propellant into the bore. (We are not breaking Newton’s law here: total recoil may be similar but the initial rearward gun acceleration will be slower and that is what the shooter is most sensitive to.)
Hence, in general, when considering identical usable case capacities, bottlenecked cases are vastly more efficient than cylindrical cases and progressively shorter bottlenecked cases are progressively more efficient. Modern designs are simply getting closer to the ideal, where the case body is sufficiently short so that very little unignited propellant follows bullet into bore.
Barrel Life Considerations
A complication exists regarding barrel life. Consider the 243 Winchester and 6mm Remington. The 243 has about 4% less usable case capacity. It also works at a slightly lower pressure. Shoulder angle is also slightly steeper. Each of these characteristics is recognized as being beneficial toward increasing barrel life. However, the 243 case neck is significantly shorter, which evidently makes all the difference.
Ballisticians have long recognized two significant things when comparing these cases. First, the 6mm is always well behaved — no surprises; conversely, the 243 is notorious for generating unexpected results, including unexplained pressure spikes. Second, inexplicably, the 6mm offers significantly greater barrel life, some have reported a 2:1 advantage!
The only reasonable explanation for the latter fact (which may also explain the former quandary) is that the long neck of the 6mm somehow protects the barrel throat. I believe this is precisely the situation. I suspect that a long case neck saps heat out of the perimeter of the propellant plug (as that plug begins to push the bullet into the bore). If the case neck is long enough, the cool brass can extract enough heat from the plug to quench the burning, at least along the front end. In this instance, a significant length of unignited propellant follows the bullet into the leade (beginning of rifling). This would allow the steel at the bore interior to cool slightly, after being heated dramatically by deformation and friction during bullet passage, before it is assailed by the full brunt of the subsequently passing incandescent propellant gasses.
Conversely, in the 243, the short case neck cannot cool the plug surface sufficiently to quench burning along the perimeter; hence, as the plug passes into the 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 the 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 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.
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.
To give some idea of the potential advantage, compare the 300 Weatherby Magnum, 30/100 SMc, and 30-378 Weatherby Magnum. Usable capacity is identical for the former two; however, with best loads, the 30/100 SMc generates 10% more velocity than the 300 WM; moreover, it duplicates 30-378 WM performance, despite having 33% less usable capacity! The trouble is, no current mainstream action will handle such a fat case as the 30/100.
Sometimes, it seems as though it will be another 150 years before we are able to bring these superior designs to the shooting public but we are making headway, through its Custom Shop, Savage is now offering the 5mm/35 SMc. This is a 20-caliber varminting number that does just about anything that the 220 Swift will do and does so without heating the barrel any more than does the 223 Rem! With time, we expect to see more of these optimized chamberings offered. SMc designs, up to 6.5mm and possibly 7mm, will work through existing actions, for 30-caliber a larger action is required.