I'm not an expert in AR gas system function, so this is just my basic understanding:
If a carbine length and mid length have the same ejection pattern, the mid length is actually receiving less gas. The bolt is moving at similar speeds because the extraction process with the midlength robs less energy; therefore, less of the force from the gas is used on just unlocking the bolt and pulling the casing out, and more of it can be used to accelerate the carrier.
If you give them both an equal amount of gas, then the midlength's BCG may travel faster. However, you still see decreased stresses and strains during extraction with the mid, so the recoil would probably not be worse. On the upside, you reduce bolt wear during unlocking (which I imagine is more severe than the wear when it locks and goes into battery faster) and increase reliability.
So, in your case, the 16" mid and 14.5" could have been receiving similar amounts of gas with correspondingly similar recoil, but the difference is that the mid would reliably cycle a wider range of ammo under worse conditions.
In effect, a midlength might achieve the same reliability with less recoil, or moderately improved reliability with slightly less recoil, or significantly better reliability with the same recoil. (Up to a point anyway, at some level you run into reliability problems from excessive carrier speed but this requires some rather severe overgassing to my knowledge.)
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