Whats the diff...
 

Between Mid Length Gas system, CAR length gas system, R4 Profile Barrel CAR length gas system on these RRA AR-15

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7" for the carbine vs. 9" for the mid-length. Biggest difference is the 9" is more forgiving of "imbalances" in the pressure curve (it's generally smoother and more reliable for a wider variety of ammunition). I believe in it enough that I only have rifle or mid-length gas systems. Got rid of my last carbine length system several years ago. Haven't looked back since.