They screwed the gas port size.
Remember the A2 stock runs a buffer spring 11.750"-13.5" with a buffer that weighs 5.2 oz. The gas port pressure is 13.5K psi in a rifle length system and runs fine as designed with a full auto carrier, millions of M-16A1's and A2's have proven this all over the world.
The middy system taps it's gas at 17K psi.
Think about this for a minute...