The AR and the Bayonet Gods
When the M4 was being developed, the bayonet Gods walked the Earth. The goal was to come up with a fully functional AR that was as compact as possible but homage had to be paid. The new AR had to mount a bayonet although the venerable pigsticker had long been abandoned by the Europeans as "barbaric". Ok, so the Bayonet Gods didn't walk the whole Earth, but they owned the U.S.
There is an optimal spot to tap the gasses of a rifle to operate the action. Any further or closer than that and it gets more difficult to tune. To make the M16 shorter, the designers had to move the gas port closer to the chamber. They went with a barrel as short as possible with the gas port located to allow a bayonet to be mounted and that barrel length happened to be 14 inches. Then they added a half inch to hedge their bets and that set the barrel and gas length for the new M4 carbine.
To make the M4 civilian legal, the barrel was by necessity, lengthened to 16 inches. Some makers pinned muzzle devices. I recall the CAR-15 clone offered by, I believe, Rock River that had an 11 inch barrel and a long CAR style flash hider permanently installed to make up the remaining 5 inches. Believe it or not, the 16 inch "M4" received heavy criticism because it could not mount a bayonet. In response, Armalite came up with what we now call the "mid-length". It was strictly to appease the Bayonet Gods. The fact it proved to be a softer shooting rifle was a serendipitous side effect. It only makes sense, however, as the rifle length system is softer shooting than the carbine and much more forgiving of a wider range of ammunition.
223 Ammo
223 ammo is not underpowered when used as intended. It makes full pressure when used in chambers designed for 223 spec ammo. The 5.56x45 chamber is designed to allow a bit more powder to be crammed into the case to make more velocity without experiencing higher than normal pressures, the same trick Roy Weatherby used in his proprietary calibers. More bullet jump is used, which gives benchresters and other accuracy buffs fits.
While NATO pressures are listed as being higher than SAAMI, they do not compare as they are measured by two completely different methods. So it isn't that 223 ammo is underpowered, it's that the 5.56x45 chamber doesn't allow 223 ammo to make full pressure


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