I agree with some of the points you raise about Sam’s post, but I do actually think logistical concerns were more important than round lethality in going with the 5.56 NATO; I even remember reading about it in some of the defense journals back when I was in the Army comparing how much 5.56 ammo you could transport vs 7.62 due to the size and weight savings.
There was actually a tremendous amount of Pentagon politics played pertaining to the M-16 and the M-14 in the 1960s.
Starting in the early days of Kennedy’s Presidency, some Generals wanted to order AR-15s and adopt them for military use, but there were factions within the military that blocked this, in part based on the claim that having two widely used rifle cartridges in the military would lead to a number of logistical issues. Since the 7.62 was seen as “more robust” because it could be used for a wider range of theoretical scenarios than the 5.56, the argument went there was no good reason to introduce the 5.56 into large scale use in the military as a second rifle cartridge.
There was also some plain old fashioned “entrenchment” that supported the M-14 and 7.62. Elements within the Army frequently referred to the M-16 as a “Matel gun”, it wasn’t the thick, beefy M-14–if you’ve ever touched a period M-14 with wood stock, these had a feel to them not unlike the M-1 Garand which was a gun that you could brain someone with as easily as shoot them.
One issue frequently harped upon is that the M-16 and 5.56 were “underpowered”, and the Army would highlight the superior performance of the M-14 when shooting targets at longer ranges. At significant range this is absolutely true–and is why for a number of years the M-14 was the base platform upon which U.S. sniper rifles were built for the military.
A number of more data-based tests consistently showed that in the actual sort of encounters infantrymen were really likely to be engaged in, the M-16 was the superior weapon and the 5.56 the superior round. There was a small deployment of AR-15s to U.S. Army Special Forces deployed in the early stages of the Vietnam War by ARPA, and the feedback from the field was incredibly enthusiastic. The ability to carry more rounds, the lighter weight of the rifle, and the performance metrics of the rounds in combat were all superior to the M-14.
The Army kept fighting, though, to the point tests were ordered conducted to settle the matter. The Army tests found that only the M-14 was suitable for use. Then the bias and legitimacy of the tests were scrutinized, and found that the Army had deliberately weighted the scales towards the M-14 in testing. Finally more unbiased and high-quality testing was done, the M-14 was shown to be clearly inferior. By 1963 McNamara had enough data that he was able to cease orders of new M-14s and start the process of getting the AR-15 ordered and deployed as the military M-16.
Now, it didn’t help that there were production and quality issues with a lot of the early batches of mass-produced M-16s, some of which were dramatically exaggerated by forces within the Army that were still opposed to the M-16–but not all of it was exaggeration, there were initial issues with scaling the rifle up to very large-scale production and reports of trouble coming in from the field. For a time, this fed into a popular narrative in the Army that the military had given them a fucked, cheap gun, trading expediency and money for the lives of our soldiers. This was really never true, but it served narratives that people wanted to serve at the time. Early performance in the jungles of Vietnam also had some maintenance issues.
One reason for some of the early troubles that hadn’t manifested in the testing and limited deployment phase, is the powder used in the ammunition was different in the mass produced 5.56 and that issued to early testers and early adopting units. All that being said–by 1968 and certainly by 1970 many of the kinks had been worked out, and polls of servicemen found a strong preference for the M-16 over the M-14; but some of the pop cultural “tales” of how terrible the M-16 was basically lived on indefinitely and are sometimes even repeated in modern times.
Also of note–the supposed tumbling action of the 5.56 round is not really why it is so lethal, it is actually specific characteristics of the round that make it more likely than the 7.62 to undergo heavy fragmentation after impact, which causes more complex wound patterns.
The AR-15 “ammo article” talked about this some years ago: Ammo Oracle (ar15.com)
Essentially the M-16 produced more serious wounds in Vietnam than the M-14, and this was often ascribed to tumbling. But the real culprit was bullet fragmentation.
Note that in the broader market there are many variants of 5.56 ammo you can purchase that can have different performance characteristics, and sometimes talk about tumbling, fragmentation and et al. get bogged down in some of these minutiae without proper understanding of variants to the ammunition and what different people are talking about / doing with them.
The U.S. Army at least at one point had 20+ different 5.56 cartridges it purchased for different purposes. There are even civilian enthusiasts who used cartridges designed more for better long-range performance, and which in the hands of well-practiced shooters can be very accurate to over 700 yards range. Most of that is neither here nor there, just note that sometimes when people argue about how the 5.56 “sucks at long distance” or “sucks at penetrating armor” or etc etc, a lot of context is necessary to evaluate these claims and what cartridge is being used.
