This was an old issue long before the Vietnam War.
In fact, the US military justified the adoption of the high velocity (for the time) .30-40 Krag on the basis that the smaller bullet would tumble and inflict wounds as significant as the much larger .45-70.
The Brits actually researched the effect of bullets tumbling in battle rifles prior to WWI and they found that the effect was maximized at .276" at the 2600-2800 fps velocities of the day. Since they referenced land diameter not groove diameter, they were in fact talking about .280" / 7mm.
That drove their decision to go with .276 as the caliber for their new battle rifle, which was however never adopted with WWI looming on the horizon and then after the war was canceled given the millions of rounds of .303 in storage. However, the experiments with bullet tumbling further drove their intention to adopt the .280 British in the FN FAL and as the standard NATO round.
However, the powers that be in the US wanted to stay with a full power .30 caliber round, in part because we had lot of tooling for the .30-06. The Brits tried to compromise with the ..280/30 which used the same head size as the .30-06 and could have been made on existing tooling, but we stuck with the .7.62x51mm NATO/.308 Win - and forced the choice on the rest of NATO.
Then, to add insult to injury, within a few years we started developing the 5.56mm cartridge as we discovered the Brits were right as the full power 7.62 was a poor choice for full auto weapons.
In order to justify the adoption of the much smaller caliber, the military pointed out logistic advantages of the smaller round, but also dusted off the old "bullet tumbling" argument to show that the smaller 5.56mm round had sufficient lethality.
There were some problems with that however, and they only got worse over time.
First it was a bit misleading as the M80 ball ammo used in the M14 tumbled at reasonably short ranges as well.
Second, Eugene stoner used a Sierra designed bullet in the original .223 cartridge to achieve better retained velocity to pass a 300m penetration test, as well as to reduce stability and maximize the fragmentation and tumbling effects of the bullet at normal combat ranges.
However, when the military put the round in production they used a shorter Remington designed bullet that lost much more velocity. This created the pressure problems with the XM193 round as that lower BC bullet had to be driven faster to meet the penetration standards at 300m. The shorter bullet was also more stable and would not tumble at as long a range.
Third, the M16 was designed to use a 1/14" rifling twist which barely stabilized the 55 gr FMJ bullets used. In arctic testing it was not adequately stabilizing the bullet so a faster 1/12" twist was adopted, which again reduced the tumbling range.
Fourth, generally speaking, the shorter Remington designed bullet in the M193 ball round needs a velocity of about 2700 fps to fragment. This fragmentation effect was an added bonus over and above just tumbling. The original 20" M16 and M16A1 could generate that 2700 fps velocity out to a pretty credible 190-200 meters. However that range gets shorter as barrel length gets shorter.
Fifth, the adoption of the M855 round using the 62 gr SS109 projectile reduced the muzzle velocity in the 20" M16A2, which also shortened the fragmentation and tumbling ranges of the combination. That only got worse as the 14.7" M4 was adopted give it's substantially lower muzzle velocity. Worse, the 1-7 twist in the M16A2 was adopted to stabilize the much longer M856 tracer round, and was over kill for the M855 round - where 1-9 was sufficient. This over stabilization doesn't help.
The end result is that the original 190-200m fragmentation range for the M193 / M16A1 combination has decreased significantly to only 45-50 meters in the M4.
The round will still tumble without fragmenting down to about 2600 fps, but that only adds another 50m or so.
----
Barrel lengths and ranges that will still produce 2700 fps.
20” Barrel
M193 190-200m
M855 140-150m
16” Barrel
M193 140-150m
M855 90-95m
14.7” Barrel
M193 95-100m
M855 45-50m
11.5” Barrel
M193 40-45m
M855 12-15m
----
Soft point bullets on the other hand are generally regarded as a more reliable wounding mechanism due to greater penetration, so for law enforcement and civilian uses where they are allowed, they are a much better option when range is a factor as they are less dependent on a minimum velocity.
In that regard, I have no serious reservations about using an AR-15 with a good soft point bullet at any reasonable range.
However, given that self defense shooting also means very short ranges, I'm also not concerned about the velocity loss in 16" barrel, and in fact my go to carbine has an 11.5" barrel which only generates about 2750 fps at the muzzle. I'm also not concerned with using M193 ammo either for home defense as at social shooting distances it will fragment just fine.
