well if it was in forge it would sort of be okay. It would, for obvious reasons, never make it to a competetive matchmaking map. It would just be useful for minigames and stuff or maybe a spartan laser in BTB if it went through some serious playtesting and balancing.
I would be fine with this but for most weapons it would be simply stupid (pistol).
I imagine that a Spartan Laser mounted on a turret would be pretty cool, as would a concussion rifle, a sniper rifle, and a grenade launcher (which I believe isn't returning). It could also work on a competetive level if the turret was placed in a high risk location and was given a small clip size or low rate of fire.