Both my short barrels are cut to 11.5", only because the place that cuts them for me can't get them into the CNC lathe any more (something to do with the HUGE chamber end). My 6.8 runs fine on setting 2. My 5.56 I have to leave on setting 4. I will probably start opening it up 5thousands of an inch until it runs a little better. Not saying that it doesn't run good, I just want to run it on a lower gas setting.
If you have a light barrel make sure if you have it cut yourself that the person doing it knows what they are doing. After you thread the muzzle for the flashhider there isn't much shoulder left. Especially if you are shooting a can, it is easy to get the threads crooked just a little bit and not have the shoulder to help seat the FH. Last thing you want is a baffle strike.