The terminal effectiveness of the 5.56 NATO has been the target of many complaints and the subject of many projects for remediation through research and development. The earlier article (“Why a New One?” link here) posited that using a longer cartridge that could fit within the magazine of rifles originally designed for the 7.62X51 NATO could effectively remedy the problem. The claim was that cases with water capacities of 40 to 45 grains of water may be able to push these bullets fast enough to provide the potential for penetration, perforation, and wounding to be seen as viable replacement candidates for the 7.62X51. Furthermore, we can obtain this internal volume in 51 mm long cases with body diameters on the order of those of the 6.8 mm SPC and the 6.5mm Grendel and, possibly, even smaller. These smaller diameter cartridges would allow as many as 25 to 26 rounds in a magazine of about the same size as the 20-round 7.62X51 magazine.
This installment looks at the methodology to be used in assessing potential terminal effectiveness as a function of bullet diameter, weight, and velocity. To start, the suitability of a cartridge for military applications depends on some factors not normally considered important in sport shooting. Cartridge size, weight and full auto controllability have significance in addition to point blank range, wind drift and terminal effects. The alternatives listed here will likely have point blank ranges that are all very close to those of the 7.62X1 mm NATO. We will also see that heavy bullets are the most likely candidates in each caliber. The resulting high sectional densities and implied high ballistic coefficients will serve to minimize wind drift.
The magazine size and weight should be less than those of the 7.62X51 NATO because the cartridges under consideration will be smaller in diameter and weight than the NATO cartridge. Rifle recoil will be somewhat greater than that of the 5.56 NATO but less than that of the full-power cartridge. Fortunately, the advent of burst limiters keeping the number of shots to 2 or 3 make controllability on full auto easier to maintain.
Terminal Effects Methodology
Turning to the key terminal effects, we note that the science of penetration and perforation has evolved significantly over the last couple of centuries. We have moved from “gee it went through!” to empirical relationships to algorithms reflecting the underlying physics to all up first principles calculations (e. g., hydrocodes) of what happens to a target and penetrator.
While the hydrocodes come closest to predicting actual behavior, short-form algorithms remain useful for key phenomena and describing broad trends. The most successful formulas found in the literature tend to use kinetic energy density for characterizing perforation and momentum density for penetration. An example of these reports is from the Swedish Defence Research Agency “A model for rigid projectile penetration and perforation of hard steel and metallic targets” by Wijk, et. al. (URL cite here). This report is cited primarily because it is readily reached through the world wide web and is consistent in the key parameters with those of many other reports.
Clearly other factors such as strength of target and penetrator, penetrator shape, angle of target to bullet flight, etc. have strong influences on the details of actual events. In the end, when we hold all of these other factors constant and vary just the impact velocity, diameter, and length of the impactor, we find that the kinetic energy density and momentum density best describe the trends for perforation and penetration.
What do we mean by “energy density?” Using to a little bit of math, we can describe kinetic energy density with the equation:
Where π is the constant Pi (=3.14159…..).
Similarly, momentum density is given by the formula:
Let’s turn to the potential effectiveness of the bullet at preventing an enemy combatant from doing his duty. One measurement of this potential is the nominal size of the permanent wound channel created when the bullet passes through. The size of the permanent wound channel depends largely on the penetration depth and the area swept out by the bullet through crushing or cutting. For bullets that are stable and penetrate nose on, this area is effectively the diameter of the bullet. The area grows as the bullet tumbles, breaks up1 or mushrooms.
We see from the discussion above that penetration depth is a function of the momentum density. How can this expression be expanded to reflect trends in wound channel dimension as the bullet, mushrooms, tumbles, or breaks up? The Hornady HITS™ method (http://www.hornady.com/hits/calculator) appears to offer a simple appears to offer a simple approach to capturing the trends. The method is a somewhat new approach to characterizing the lethality of standard hunting bullets. The method is gaining some popularity (See, e. g., http://www.chuckhawks.com/rifle_hits_table_syn.htm). The wounding potential embodied in the HITS™ method is the momentum density times the weight of the bullet. The momentum density is a measure of the length of the wound channel. The added weight factor can be viewed as suggesting trends in wound channel diameter resulting from expansion, and in full metal jacketed bullets, tumbling. Thus the HITS™ score serves as a general indicator of trends in wounding potential:
The factor “k” converts a very large number into a non-dimensional one that is easy to think about. We assert The HITS™ score is equally useful for comparing the wounding potential for full-metal jacketed bullets in combat.
Baseline Considerations
Federal American Eagle™ 150 gr FMJ ammunition corrected from 24 inch barrel length to 20 inch barrel length is used to characterize the penetration and perforation performance of the M80 round to allow a consistent 20 inch barrel length for comparisons. We’ll look at performance in shorter barrels typical of CCQ weapons in a future article. The question asked will be “What muzzle velocity is needed to equal the baseline at the specified range.?”
Key parameters for 150 gr FMJ ammunition
| Bullet Diameter | 0.308 inches |
| Weight | 150 grains |
| Muzzle Velocity (24″ bbl) | 2820 ft/sec |
| Muzzle Velocity (20″ bbl) | 2740 ft/sec |
The Hornady HITS™ methodology suggests that a 150gr .308 standard hunting (not full metal jeacket) bullet in this caliber at 2740 feet per second is suitable for non-dangerous game weighing well over 300 pounds. Very few combatants weigh this much even when carrying a full combat load. Requiring a match against the 7.62 for all performance categories would make the resulting cartridge look like the .308 Winchester — a rather uninteresting outcome with perhaps more lethality than needed.
The Hornady HITS™ methodology suggests that a 150gr .308 standard hunting (not full metal jacket) bullet in this caliber at 2740 feet per second is suitable for non-dangerous game weighing well over 300 pounds. Very few combatants weigh this much even when carrying a full combat load. Requiring a match against the 7.62 for all performance categories would make the resulting cartridge look like the .308 Winchester — a rather uninteresting outcome with perhaps more lethality than needed.
Looking at the 5.56 NATO, we see that few people have questioned the short range performance of the M855 green tip ammunition. The complaints are that this performance is not available between 200 and 600 meters. By requiring that the new cartridge demonstrate the same wounding potential at 600 meters as the M855 at 24 meters from the muzzle, we assure ourselves that we will see a significant improvement in performance at all ranges. The the 62 grain M855 green tip ammunition has a velocity1 of 3025 ft/sec 24 meters from the muzzle.
We will see that the only bullet diameter where the arbitrary doubling of wound channel metric influences the outcome is the 5.56mm (0.224). The bulk of the bullet diameter, weight, and velocity suggestions are driven by the perforation and penetration criteria. The methodology also suggests that, in at least some cases, the penetration and perforation criteria result in bullet weights and velocities with potentially greater potential for preventing completion of duty than the 7.62 NATO at the longer ranges.
1 Fackler, Martin, MD, “What’s Wrong with the Wound Ballistics
Literature, and Why” ( http://rkba.org/research/fackler/wrong.html ).
2 http://www.inetres.com/gp/military/infantry/rifle/556mm_ammo.html
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