Very complicated to accurately measure. Very difficult to even guesstimate. And yet, it's exactly what would need to be measured to have any quantifiable data on what does and doesn't reduce recoil (felt recoil, if you will) to the shooter.
I personally haven't experienced this reduction in recoil others have attributed to the mid-length system. I think the whole concept of reducing recoil with the gas system ignores two constants:
-Impact force from the initial detonation is going to be significantly higher than the subsequent reciprocation of the buffered carrier. The cartridge blows and shoves bullet and hot gas in one direction; equally opposing the entire weight of the weapon, via closed/locked bolt lugs, back into the shooter's shoulder, well before the gas system is engaged.
-You have to punch the carrier hard enough to cycle the weapon, and that means you simply cannot create a huge reduction in energy transference and still have a functioning semiautomatic AR15. Regardless of how you time it, or add stretch to the rope so-to-say, you still have to hit the carrier with enough force to drive it all the way back. Carbine or midlength, it's the same force required to do this.
I think correctly balancing the ratio of dwell:gas is one of a handful of important components to a reliably functioning rifle, but I will not buy claims of reduction of recoil, "perceived" or otherwise. Here, I believe MistWolf and I come to full agreement (if by different paths).

