Almost since introduction the 5.56×45 NATO has exhibited the qualities of Schrodinger’s Cat, simultaneously being an ineffective battle round while simultaneously being terribly effective to the point where people keep trying to ban it for civilian use. Thousands upon thousands of words, and hours of argument, for both sides have been spent on the subject.
The 5.56 naysayers routinely pull out ballistic tables and show all sorts of lovely numbers about energy in foot pounds remaining at range: http://usacac.army.mil/CAC2/MilitaryReview/Archives/English/MilitaryReview_20120831_art004.pdf and it should be noted that this article makes the “post hoc” fallacy that the 5.56×45 is inadequate because the M14 EBR program was used. The author makes no analysis of what the mix ratio was, and how those rifles were employed, and other tactical considerations. In Afghanistan the SEALs of Seal Team 10 often considered the 7.62×51 inadequate as a sniper rifle platform since they had the 300 Win Mag as an option due to the open ranges in the flat desert areas and mountainous areas, but that is another story for another time. Also, it should be noted that the Russians kept the SVD in the Platoon level, basically doing the same thing that the M14 EBR rifles did for the US in the War on a Noun.
The other side focuses more on the “terminal” part of terminal ballistics which is where the bullet meets flesh. At that point the question about “how much energy” is less important than “is there enough energy to do the job?” And you find analysis like this: https://www.americanrifleman.org/articles/2014/5/21/testing-the-army-s-m855a1-standard-ball-cartridge/ and http://www.thefirearmblog.com/blog/2010/07/19/in-defense-of-the-5-56mm-nato-round/#more-14128 All these points are good points, but fail to address the question of “can GI Joe actually put the rounds on target?” and “is the collapsible buttstock of an M4 combined with a nine pound trigger pull really the best choice when we are relying mostly on GI Joe’s marksmanship ability to ensure lethality?”
The terminal ballistics crowd has always had a problem with the M855, as it was an unreliable “tumbler” in flesh or ballistic gel. This gives rise to where you can have a completely adequate cartridge, but a bullet inadequate for the task. The common wisdom gave rise “use the 55gr M193 bullet for home defense as it is yaw independent at home defense ranges” where the M855 was yaw dependent (and that is still valid advice if you feel you need an AR-15 for home defense).
Now up front I will posit that all bullets are potentially lethal, even the 25 ACP. But biology is a very important factor, giving rise to such sayings as “I’d rather by shot in the foot by a 50 BMG than in the back of the head by a 22 short.” Where a bullet impacts on an organism matters, since we have yet to perfect a weapon system that can incapacitate with a pinky toe shot.
What the “ballistics table” crowd fails to comprehend is that we shoot a LOT more 50 BMG than the “one shot, one kill” crowd would like, and a lot more 7.62×51 fired as well. This is where the “terminal effects” crowd has a bit of a leg up, as having more chances to shoot someone who is trying not to be shot by using cover and concealment, is a good thing. Where the “ballistics table” folks have a leg up is that nearly every lethality study shows that as caliber dimension increases, the time from bullet impact to organism death shrinks (at least based on the hunting of large mammals). Being able to carry more ammo is a good thing, but bigger ammo is better at turning large animals into meat.
The lessons learned from Iraq and Afghanistan is that the 7.62×51 is not a magical death laser, nor is an M4 an impotent spitball thrower. I knew a Scout Platoon that was working the ridgelines in east Paktika and were outfitted in all 7.62×51 M14 EBR rifles, which made tactical sense for them as they didn’t bring an M240 machine gun with them to do their job. I also knew a sniper team that did urban work in Iraq and preferred a suppressed rack grade M4 tossing out Mk262 Mod1 ammunition when they could get it, M855 when they couldn’t. I know that adapting your kit to fit your environment is a radical idea to the “one solution for all problems” crowd, but one size never truly fits all. Sometimes “one size fits most” is purely wishful thinking.
Taking a step back in history to the 30 Carbine, we can clearly see that simply increasing caliber isn’t always the easy answer. The 30 carbine is the same diameter as the mighty 7.62×51 NATO, but had quite the reputation as a wounder, firing a round nosed pistol style bullet rather than a long, spitzer bullet. The 30 Carbine also developed a bit of a reputation for punching neat holes but not stopping people, very much the same as the M855 green tip ammo did. Both the Army and the USMC addressed that shortcoming with M855A1 and Mk318 respectively, both of which are “barrier blind” and both of which have more consistent terminal effects in tissue. Add a soft point bullet to the 30 carbine, and all of a sudden it is a much more effective round on living critters.
So I don’t expect to see the 5.56×45 replaced any time soon as some simple math will show why. A marginal increase in weight and diameter also gives a marginal increase in “lethality” but with fewer chances to achieve effects. If you have a 10% chance of killing someone with 5.56 and 15% chance with a 6.XX round, you have to fire 10 rounds with 5.56 to achieve the statistical 100%, and you have to fire 7 rounds with the 6.XX round. So you can make the argument that you could carry 30% less ammo to achieve the same lethal effects, but anyone who has been in a sustained firefight will tell you that you are an idiot for cutting 30% of your ammo supply (and no, no one wants to go to a 22 WMR platform to carry lots more ammo since there is a bottom level of effectiveness at range and none of the rimfires are going to cut it at 600 meters). Now if you could make a 6.XX round that weighed the same as the 5.56 on a per cartridge basis, you would actually increase the effectiveness of the individual weapon system. But as of right now, that’s not the case, all the contenders to dethrone 5.56×45 as the standard issue round weigh significantly more.
Here is a real world comparison between the 5.56×45 and the 6.5 Grendel.
5.56 brass: 93gr 6.5 Grendel brass 117gr (+24gr)
5.56 powder 26gr 6.5 Grendel powder 32gr (+6gr)
5.56 bullet 62gr 6.5 Grendel bullet 115gr (+53gr)
5.56 weight 181gr. 6.5 Grendel 264gr. For comparison, the 175gr Sierra Match King bullet shot by the M118LR or Mk316 Mod0 sniper round is only six grains less than a total 5.56×45 cartridge, and a lowly M80 ball round comes in at a whopping 392 grains. Clearly the 6.5 Grendel falls between the 5.56×45 and 7.62×51 in weight, but comparing match bullet to match bullet, it also falls there for performance as well.
Total weight cost of the 6.5 Grendel over 5.56×45, 83gr per cartridge. That’s essentially a 50% increase in weight which doesn’t justify what is essentially a 5% increase in lethality per shot. The 6.5 Grendel would have to be 100% better (ie 20% guaranteed lethal per shot) to justify a 50% increase in weight just to get to the same weight per soldier to enemy killed ratio, and you would still lose 1/3rd of your total shots for a sustained fire fight (5 shots per kill rather than 10, so the 6.5 grendel would save 2.5 rounds worth of 5.56 weight in that scenario, unfortunately it doesn’t in real life). In real life that Infantryman still needs to be able to sustain “suppressing fire” to allow his buddy to move to that position of advantage for the kill shot.
That is the sort of math that the caliber wars are really fought on, not the ballistic tables and not the terminal ballistics, but in how much firepower can an American grunt bring to the fight, how many shots can he take to make the other side keep their head down while his buddy maneuvers into a position of tactical advantage for a kill shot. Maybe new polymer cased telescoping ammo will change that calculus, but until then there is no reason to leave the 5.56×45 for something heavier.
That doesn’t mean that people selling something heavier aren’t going to try other methods to sell their products such as “appeal to emotion” or other logical fallacies. One of the rallying cries of the “ballistics table” crowd has always been “Dead Infantrymen!” caused by failures of either the M16/M4 or the 5.56×45 round.
A good example of the “Dead Infantrymen!” appeal to emotion is typified by the MG(ret) Scales hit piece from a few years back where he blamed the dead at the Battle of Wanat on the M4 platform: https://www.sofmag.com/major-general-retired-robert-h-scales-blasts-the-armys-lousy-rifles/ (however you have to question the validity of opinion of someone who says that the “Remington 270” would be a valid upgrade from the 5.56×45, after all MG Scales was NEVER an Infantryman so his knowledge of close combat was much more academic than practical).
To answer that Ivory Tower hit piece written by the former commandant of the Army War College, read the late Hognose’s (SF long tabber with real experience in the same time frame as MG(ret) Scales) analysis of the Battle of Wanat: http://weaponsman.com/?p=20023
Now, all of this is based on military use. If you are a prepper and settled on 308 Win, 357 Mag, 30-06, or even 30-30 or a muzzle loader with a shotgun, stick with it. None of these combat oriented military considerations apply to your situation (unless you plan on using military small unit tactics to attack other groups using small unit tactics). What you chose based on your situation is much more appropriate than “well the military uses it so it must be good!” level of magical thinking (the military uses crap built by the lowest bidder gained through the worlds most dysfunctional acquisitions process).
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EDIT: Andrew Tuohy of Vuurwapenblog addresses the tomfoolery of MG(ret) Scales. http://www.vuurwapenblog.com/general-opinion/firearms-general/general-scales-is-still-wrong-about-infantry-rifles/