Is there any challenge/title/emblem that you get for kills with a fixed gun emplacement like the ones found on Bakara or Liberation? In previous games there was, but I've gotten a 5+ killstreak, and it seems like there was no benefit.
Yes. The MG Master challenge makes a return, in the last set of unlocked challenges. It requires a 5-kill streak with the mounted miniguns. You might not be able to complete the challenge before unlocking it at level 70.
You're lucky to be playing on a console. We PC players only get the machine gun on Bakara to attempt that challenge with... :(