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“Flu medicine Groggy mind
... Infantry rifles from WW1 & ww2 fire a more powerful cartridge in terms of range, and knockdown power than those used today. The advantage of an M-16 is that it can fire much faster, is easier to handle and lighter.
A semiautomatic AR-15 does not fire any faster than an M1 ... 7.62X63 bullets have far more stopping power. You can make an argument that the M1 is a more powerful rifle than the AR-15 ...”

Under the effect of flu remedies, and the flu itself, I’m lucky if I can find the keyboard. May your recovery be swift and complete.

Military rifles of the World Wars fired those large, powerful cartridges because those were adopted in the period 1886-1906: superior long-range performance and great penetration against massed native warriors in the colonies was important. Several nations were planning to rearm with different rounds, but the First World War negated all that. Similar plans went down for similar reasons in the late 1930s.

“Knockdown power” and “stopping power” are tossed about frequently, and widely believed in by the gun-owning public in the USA, but no rigorous definition exists and they defy quantification. They are by law not allowed to be used by most national military establishments (including US Dept of Defense) in determining the relative effectiveness of small arms.

The optimum cyclic rate for full-auto arms is about 600 rds/min, according to most US ordnance authorities. Full-auto small arms in US service have fired at various rates, from 350-400 (rds/min BAR M1918AR, Submachine Gun M3), to 600 rds/min (M60 Machine Gun) to 700 rds min (Thompson Submachine Gun) to 650 rds/min (M14) to 900 ds/min (original M16) to over 1000 rds/min (M240 Machine Gun, some variants). Practical, in-action rates of fire are much lower: 40-60 rds/min maximum, chiefly to conserve service life and avoid compromising functional reliability.

“Power” is also a term of little precision, though it can be quantified better. Kinetic energy developed at the muzzle is a more common quantity used in comparisons. 5.56x45mm NATO thus develops about half the energy that 7.62x51mm NATO does.

The tiny 5.56x45mm NATO cartridge - tiny by World War standards - was adopted as the best compromise, between effective range and full-auto controllability. In sum, the average soldier cannot control a standard-size rifle in hand-held full auto mode, firing 7.62 NATO; 5.56 NATO (at first merely the US-only 5.56x45 M193) can be controlled. By the 1950s, most US Army field-forces leaders were convinced every soldier required a rifle capable of full-auto fire. Army Ordnance Corps officers and a few other senior people did not accept that, insisting that 7.62 NATO was necessary. Eventually, Ordnance got overruled. Firepower was redefined as shots per minute.

Given the treaty limits on bullet construction, it’s been argued (by Peter G Kokalis, no less) that 5.56 NATO is actually more effective than 7.62 NATO and the earlier 30M2 (7.62x63) - under ranges of 100m. Inside that distance, the larger, heavier 7.62mm bullets punch right through a human target and continue downrange. But the 5.56mm M193 goes unstable on entering human tissue and fragments, expending all energy inside the target. Beyond that distance, effectiveness is reversed.

The advent of “intermediate” cartridges for military use cannot be considered without acknowledging the changing nature of the battlefield, from 1900 through 1918, to 1945 and later. The prior mission of an infantryman was to pick out and individual enemy soldier and hit him with aimed fire; shot-for-shot effectiveness and long effective range were key attributes. By 1945, long-range fire from infantry was less important in the killing role: the infantryman was no longer considered in isolation, but had become part of a combined-arms team, each element of which had to operate in coordination with each other element to get the best results. A heavy rifle firing a substantial bullet to effective ranges of 2000m or more was found to be overkill - in a literal sense. Bullets of 2/3 the weight fired to only 75 percent the velocity of the earlier, large rounds were found to be “good enough.” Thus the assault rifle came to be.

As the late Jeff Cooper put it, “The infantry was no longer expected to kill enemy soldiers, it was expected to find enemy soldiers, then call for supporting fire from heavier weapons. No longer did the infantry need a heavy rifle nor skill at marksmanship; what it needed was aggressiveness, mobility, and a good telephone.”