Synopsis  I. Why a New One?”  II. Terminal Effects  III. Required Velocities  IV. Velocities — Summary 
The terminal effects note gives us the tools for initial assessments of the claim that intermediate cartridges could have effectiveness close enough to that of the 7.62X51 NATO to be useful. This note applies the methodology so we can see what the cartridges have to do.
Battle Rifle Potential
Sierra Matchking^{TM} bullets were used to provide a consistent cross caliber comparison of ballistic coefficients. These bullets have slightly higher ballistic coefficients than the full metal jacket offerings in each caliber but the higher performance should be attainable with current volume production technology.
We can solve the three equations shown in the terminal effects installment to easily calculate the threshold velocity for each criterion:
Perforation:  V_{e}=(V_{b}*R_{e}/R_{b})*(M_{b}/M_{e})^{1/2} 
Penetration:  V_{p}=(M_{b}/M_{p})*V_{b}*(R_{p}/R_{b})^{2} 
Wounding:  V_{w}=(M_{b}/M_{w})^{2}*V_{b}*(R_{p}/*R_{b})^{2}) 
Where:  (M_{b}, V_{b} R_{b} are the weight, velocity, and radius of the baseline bullet and the subscripts e, p & w respectively refer to the perforation, penetration, and wounding criteria. 
The 150 gr FMJBT velocity of 2740 ft/sec at 24 meters from the muzzle was used to calculate the three velocites: perforation, penetration, and wounding potential. The highest of these defined the minimum velocity for equaling the 7.62 NATO lethality. The same procedure was applied using the residual velocity at 600 meters, and then the muzzle velocity needed to get this residual velocity was calculated. The higher of the two muzzle velocities became the required muzzle velocity for this investigation. This assures that the required effectiveness at intermediate ranges is attained. A partial set of results is displayed in Table 1., which recaps the velocities needed to at at least equal the terminal effects of the 7.62 in all categories from the muzzle (15 meters) out to 600 meters.
Starting with the 120 grain column, we see that the sectional density is 0.246 and that the velocity needed to match the M80 7.62X51 kinetic energy density at 600 meters is 1461 feet per second. This happens to be the highest velocity of the three categories to match (KE density, Momentum Density, and wounding potential) so it is the velocity at which a 120 grain bullet needs to arrive at 600 meters to equal or better a M80 7.62X51 in energy and momentum density plus have at least the wounding potential of the M855 5.56 NATO at 24 meters.
The 120 grain bullet needs a muzzle velocity of just under 2700 ft per second to arrive at 600 meters with 1461 feet per second. At 600 meters, the bullet at 1461 feet per second exceeds the wounding potential requirement by about 30 percent and has about 80 per cent of the wounding potential of the M80 7.62X51 at that range.

Bullet Weight (gr) 120 130 140 Sectional Density 0.246 0.266 0.287 Velocity to Match KE Density 1461 1404 1353 Velocity to Match Momentum Density 1401 1293 1200 Velocity to Match M855 wound. Pot. 1122 956 824 Matchking ****Matchking Matchking Required Muzzle Velocity 2700 2525 2450 Wound Potential vs M855 1.3 1.5 1.8 Wound Potential vs M80 0.8 0.9 1.3
Looking at the 130 grain bullet, we see that a muzzle velocity of 2525 ft/sec is needed obtain the threshold level of performance. The 140 grain bullet requires a velocity of 2450 ft per second at the muzzle to satisfy the perforation potential at the muzzle. This gives a 600 meter velocity that is about 150 ft per second higher than the 1353 ft per second shown for the controlling velocity in the table and The higher velocity gives almost twice the wounding potential than the 24 meter level of the M855 and is about thirty per cent higher than the M80 round at 600 meters. This could be a very capable round.
Similar discussions can be made for .224, 6mm, and 25 caliber bullets but the 6.5 is the only caliber that appears to have the potential for exceeding the 7.62X51 in all three categories 600 meters. This conclusion might be different if lower drag bullets than the Matchking^{TM} could be made available for military applications. It appears that currently available bullets in .25 caliber are not as aerodynamically efficient as the other three calibers but the caliber would also perform well if lower drag bullets are available.
Additionally, the lighter bullets suffer from a double whammy in that they need to have a relatively high velocities to develop the terminal effects and they slow down faster. This means that the muzzle velocities have to be very high, and actually force a larger case volume to get the velocity potential.
Table 2. shows selected bullet and threshold velocities for the four calibers considered. The Ammoguide.com database was searched to find the smallest existing cartridge that could give the threshold velocity in a 20 inch barrel. The velocities were corrected to reflect the 20 inch by assuming a loss of 30 ft per second for each inch of length in the database greater than 20 inches. Hence the conclusions are approximate and preliminary. The .223 Remington can push the 90 grain at more than 2500 ft/sec from a 24 inch barrel but the bullet must be seated to an overall cartridge length of 2.38 inches – 0.12 inches longer than can fit in the magazine of an AR15. When corrected to a 20 inch barrel, the velocity doesn’t meet the required velocity, so the .223 was not included for both reasons.

Bullet and Muzzle Velocity Surrogate Volume 90 gr .224 @ 2474 ft/sec 107 gr .243 @ 2560 ft/sec 6mm BR Remington 39.4 120 gr .257 @ 2800 ft/sec 140 gr .264 @ 2450 ft/sec 6.5 Bench Rest Magnum 44.7
The heavy bullets, however, appear to be effective with case volumes of 40 to 45 grains of water, possibly smaller as suggested by the .223 Remington appearing to have almost enough powder capacity.
Machine Gun Potential
Noting that at least two of the threshold level bullet and velocity combinations equal or exceed the 7.62 NATO lethality at the muzzle and at 600 meters, the question of whether these or other combinations could do the same out to the maximum effective ranges of machine guns. The maximum effective range for the 7.62X51 NATO in machine gun applications appears to be about 1000 to 1800 meters depending on gun and mount with tracer burnout at about 900 meters (see, e. .g, http://www.specialoperations.com/Aviation/Black_Hawk/Aircraft_Profile.htm and http://www.statemaster.com/encyclopedia/M240machinegun). We would like to be sure that the effectiveness of the alternative cartridge is at least that of the NATO round at the longer ranges. This at first seemed a reasonable expectation for at least some of the bullets because their ballistic coefficients suggest an increase of velocity compared to the 7.62X51 as range increases. This tells us that the penetration and perforation potentials will increase from equal at the muzzle for these bullets to a significant improvement at the longer ranges. The wounding potential compared to the 5.56 will also increase. For machine gun applications, however, the 7.62X51 is the norm, so how do our bullets compare?
Figure 2. is a plot of the ratio of the wounding potential of alternative bullets to that of the 7.62X51 NATO round as a function of range. The .25 caliber wasn’t considered because the ballistic coefficients were too low to give a chance for overtaking the wounding potential of the standard round. As it happens, both the 130 gr and 140 gr bullets in the 6.5 mm category had ballistic coefficients with the desired potential. Similarly, the 107 gr 6mm and 90gr .224 bullets had good ballistic coefficients so their relative wounding potential was plotted as well.
We see that the ballistic coefficients keep the alternative bullet velocities gradually increasing relative to the 7.62X51 with the result that the wounding potential also increases relative to the baseline. Indeed the 140 gr 6.5 mm bullet with a muzzle velocity of 2450 ft/sec has a greater wounding potential at the muzzle, increasing to almost thirty per cent greater at 1800 meters. The 130 gr with a a muzzle velocity of 2525 ft/sec starts out with slightly below and increases with range, but is always within about 5% of the standard. The 107 gr 6mm stays within 8090% at all ranges, while the 90 gr .224 bullet stays within 6575% at all ranges.
Conclusion
We see from this information that heavy bullets in 6mm, 25 caliber, and 6.5mm driven by intermediate capacity cartridge cases have the potential for replicating the perforation and potential of the 7.62X51 NATO while maintaining an adequate wounding potential. Surprisingly the 140 grain 6.5, when meeting the threshold perforation potential exceeds the wounding potential of the 150 gr 7.62X51 bullet. This results from the very high sectional densities of these bullets.
We will need to do additional analytic assessments to verify the required case capacities, comparative performance out to the maximum ranges of 7.62mm machine guns, and so on. These assessments will determine which caliber has the best potential for replacing both the 5.56mm and 7.62mm NATO rounds. Basic physics arguments suggest that the 6.5 caliber will need the largest case. Consider the 90gr .224 and the 140 gr 6.5 bullets. They need about the same muzzle velocity, so the heavier bullet carries about 50% more kinetic energy so we know a larger case will be needed compared to the .224 diameter. Key findings may be experimentally validated at any time that someone has the inclination and resources.
Acknowledgements: My thanks to Tom Bender, a long time film industry armorer, who helped provide the inspiration and encouragement to look at the alternate point of view: Frank DeSomma of POFUSA (http://www.pofusa.com/main.htm) for confirming the need to look at new cartridges now and then, particularly as technology (this time in the form of better powders) creates new opportunities; Charlie Cutshaw, a military firearms author, for pointing me at Dr. Martin Fackler’s excellent discussions of wound ballistics; and Stan Crist, an acknowledged firearms expert and writer, for some very interesting email conversations and nudges as this story unfolded.
Synopsis  I. Why a New One?”  II. Terminal Effects  III. Required Velocities  IV. Velocities — Summary 