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7.62x51 has longer range, flatter trajectory, more energy, and more accurate than 7.62x39. None of those things are my question.
My question is, at ranges less than 150 yards, how much difference would there be in wounding characteristics against human adversaries for these 2 cartridges?
Well, since you're looking for a way to quantify the difference in performance at range for the 7.62x51 M80 (147-grain FMJ @ 2,800 fps) and the 7.62x39 M67 (123-grain FMJ @ 2,329 fps) why not use what the US military has used for that same purpose since the early-1960s? The SLV (survivability, lethality, vulnerability) equations discussed and applied in this post are still in use by the US military today in the ballistic insult subroutines contained within ORCA/MUVES-S2 software that has been used to ''match'' the lethality of our small arms munitions to that of our adversaries over the last 50+ years with great success. To the credit of this long-proven modeling approach is the development and fielding of the 5.56NATO M193 which was developed (using these same models) to have the same lethality as the Soviet 7.62x39 M43 that allowed our troops to carry considerably more ammunition afield while still matching our adversaries' munition's capabilities.
The US military which tends to evaluate the lethality of such munitions in terms of probability of incapacitation— or P(I) —and still relies upon the P(I) model that was initially developed in 1968 by Sturdivan & Bruchey at the U.S. Army Ballistic Research Laboratory (BRL) at Aberdeen Proving Grounds, Edgewood Arsenal. The BRL P(I) model is still employed in the US Army's Ballistic Research Laboratory ORCA/MUVES-S2 SLV software and is still referenced in recent research pertaining to the development and evaluation of new munitions. Assuming stable, 'nose-forward' flight through a human torso and a G7 ballistic coefficient of 0.200 for the 7.62x51 M80 and a G7 ballistic coefficient of 0.155 for the Soviet 7.62x39 M67, the velocities can be computed for use in the BRL P(I) model which are presented along with the corresponding BRL P[I/H] at each range for each cartridge.
As can be seen from the probabilities above, using the US Army BRL model to evaluate the US 7.62x51 M80 and the Soviet 7.62x39 M67 at ranges less than 150 yards, the 7.62x51 is superior to the 7.62x39 at all ranges.
Finally, using the US Army BRL ΔE15 parameter—a projectile's kinetic energy expenditure from a penetration depth of 1 - 15 centimeters—it is also possible to determine the expected time to incapacitation (in seconds)— or T(I) — for each projectile at those respective ranges:
Once again, the 7.62x51 M80 produces shorter expected times to incapacitation than the 7.62x39 M67 at all listed ranges.