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The OP has the history pretty much spot on. The rifle was design around a 55gr projectile in a 20in barrel with what we call the rifle length gas syste. When FN introduced the 62gr round with the steel penetrator it was for the M16. Because the twist required was greater than 1 in 12 the rifles were rebarrelled with 1 in 7. With the higher twist rate it was possible to begin shooting heavier bullets. Along cam Somalia and the Army decided it wanted a shorter rifle and the M4 was born. With the 14.5” barrel the gas system was modified to try and emulate the rate of fire of the original M16 using the 62 grain round. This became the carbine gas system. I think Armalite then introduced the mid. Length system for the 16in barrel to try and keep the pressure/cycling rate in spec.

It’s important to remember that the rifle was designed as a fully automatic weapon. But early on it was recognized that any more than a 3 or 4 round burst was a waste of ammunition so control 3 round burst firing was adopted and eventually incorporated in the M16A2. The basic design is overkill for a semi automatic weapon.
 
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