Whats the diff...

[ODA CDR (RET)]

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

http://www.rockriverarms.com/index.cfm?fuseaction=category.display&category_id=462

[Peregrino]

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.