About, 25 years ago, I asked some of the decision makers about this, the same thing. They knew that the carbine uppers could run with a wider span with a rifle action system on the lower compared to the carbine type system on the lower. It wasn't a secret then more than it is now. The rifle like action system worked better overall in function. It is a function of timing of events.
When asked specifically about the spring, RE length selection, and buffer mass, some interesting things came up.
The buffer mass was pushed as the most viable option at that time. I get conflicting reports of suitable tungsten weight inserts during those times being available for small or large production. Anyways, steel was accepted at the time knowing that is has the lower span of function when compared to a heavier one that may not have the ability to supply as components.
The RE length was meant to fit an AOL requirement that some wanted. I do not know where that came from, but it was a requirement at some time. On the base gun, the rifles ran worse for the requirement with less barrel length than less RE length. It was an consensus that to have more barrel length and less RE length. I do not know if this happened before or after the choice of tungsten weights was eliminated?
As for the spring? It was a modified rifle spring intended to work within the shorter RE dimensions. That functioned well enough, but less than ideal.
This was all done knowing that the rifle like action system performed better. They ended with a functional system, but with the addition of issues. I really think that the 3 ounce carbine action is a mistake for most. Even with proper porting, a 3 ounce buffer may have a lower range or span on function when considering the alternatives we have now.
Some AR's have longer gas systems to help address some base timing of events issues. A better way may or may not include that with a rifle like action system. The idea of the commercial A5, minus the internal spring, has been around since before my inital question a quarter century ago. Many different versions have been around in at least limited circulation from before my time.
I can not see in any way that the properly gassed 3 ounce carbine buffer action system can have any more of a range in function than a properly gassed version of the A5H2 system.
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