The "step" in an M4 profile barrel has to be a certain distance from the chamber end for proper mounting of the grenade launcher, which is the actual original purpose of the "step". In a 16" civi M4 profile barrel, the extra length is added "heavy" contour length between the step and the flash hider... the step is still the same distance from the chamber.
With a midlength gas system, the gas port is moved ~2" forward of where it is on a carbine gas system. I don't know the *exact* dimensions, but this would likely put the FSB into the "step" on an M4 contour barrel. Therefore you can't have the step. Putting a step forward of the FSB on a midlength gas system barrel would be even more useless than it is on a 16" civi market M4 profile barrel, since the dimensioning alone would make the step completely useless for its intended purpose.