[Rupert]

Originally Posted by Ji ji

To my knowledge the terminal performance of 7.62x51 ball is much better than 5.56x45 ball at short distance and hugely better at long.
I have my good share of biases and maybe I am overestimating some data over some other. I would like to explain this in a historical-industrial-political frame.

The standard 7.62x51mm bullet used by the US tends to just zip right through a person, and yaws well after leaving their body - it makes fairly straight 7.62mm holes in people unless it hits bone (and hitting bone and breaking up/scattering bones fragments is something all rifle bullets in the range we're discussing do). Some other countries' rounds used bullets that yawed or broke up more readily, but by and large the 7.62x51mm NATO's advantage over 5.56x45mm NATO is one of retained energy at long ranges and of penetration, not wounding ability.

As for the development, at the time the European members of NATO had reached the same conclusion that the Soviets and, and that the Germans had in WWII (and which was actually known before WWII) - that the average rifleman had no need to engage target over 300-400m, and if their unit did it was the machinegun(s) that did the work. Thus cartridges designed in the late 19th century for effective aimed fire at 600-800m and volley fire out to over a mile were massive over-powered and much heavier and more punishing to fire than they needed to be.

The US' thinking, however, was dominated by what some call 'the cult of the rifleman', and held that long ranges were necessary and useful (and that if your soldiers couldn't shoot that far they needed better training). It turns out that this is simply wrong, and not just in the jungles of Vietnam. So they muscled everyone else into accepting a new round that had the same ballistics as the old .30-06 load that was used in the M1 Garand (because newer, longer ranged loads proved problematic in it) and M1903 Springfield. It was, and is, a little more accurate and a little shorter (and slightly less versatile as a hunting round), but is really just another late 19th century full-bore battle rifle round.

Then, fifteen years after the brand new cartridge proved less-than-optimal in a very different scenario. The Army was (relatively) swift to develop and adopt a new cartridge, which is not really so new but a revamped hunting cartridge (intended for varmints, I believe).

The 5.56x45mm/.223 Remington (which were effectively the same back then, but aren't quite the same now) could well have not been used and the .222 Remington magnum used, yes. The .222 Remington was probably a little underpowered, and the .220 Swift stupidly over-bored for a military round.

This makes sense from an industrial POV. Yet I wonder if there is some other force here. The military management deemed a 7mm cartridge as inadequate just fifteen years prior, and pushed and alliance of many states to adopt its 7.62 wonder-cartridge instead; coming back to the 7mm 2500 j idea would be very bad marketing. So you choose a smaller cartridge, full of merits in its own, but not necessarily the best option.

The AR-15 was originally taken up by the USAF as a light-weight rifle for aircrews. It was not intended as a general-issue assault rifle. It was adopted by the US ground forces because they needed something other than the M14 now, and it was in service and proved to work (of course they then messed it up in all kinds of ways).

As for the bullet - the 5.56x45mm rounds of the time were carefully designed with a bullet that was barely stable (early weapons had rifling slow enough that the bullets were actually unstable in arctic conditions and the twist had to be tightened a little), so it yawed quickly on hitting anything, and at short ranges broke up.

Clever design like this was nothing new - the British .303 Mark VII had a bullet that was made longer (helping long range ballistics) by inserting a small cone of peat into the nose, which also made the bullet rear-heavy (and thus less stable) so it yawed quickly on hitting flesh. As those bullets had exposed lead bases (because of the manufacturing process) the rear flattened when they yawed, giving them a shape like a pine-seed's 'wing' and making them spin and tumble wildly. Note that this was a 'side-effect' of the ballistics and manufacturing, and therefore not illegal (and if you believe it was an accident, I have some soon-to-be-underwater beach to sell you).

If the US hadn't been fixated on having a full-power round in the late 40s, it's possible that NATO would've ended up with something in the 6.5-7mm range, of intermediate power, and would've been a lot happier about their cartridge choice for the past 60-70 years. OTOH, when the Soviets re-examined their choice of 7.62x39mm in light of NATOs adoption of the 5.56x45mm they found the light weight of the smaller round compelling and went to a similar round, rather than to something like the 6mm PPC. Note that the 5.45x39mm also has a carefully designed bullet that 'just happens' to yaw easily, and there were complaints about it being a 'dum-dum' bullet when it was deployed in Afghanistan in the 80s - it was 'too good' at wounding (note that it's actually a little less powerful than the 5.56x45mm).