I'm thinking a heavy version of the AI turret that you actually sit in, can take some damage before being destroyed (maybe 3 halberd shots to kill) but provides no cover to the user sitting in it besides the chair itself and the guns themselves covering the lower body from the side (skeletonized design) The turret would deal AV damage, with a TTK slightly faster than a traditional AV turret (maybe half a second) but with the distinct disadvantage that it does said damage via a pair a machine guns so you would be taking significantly more risk to sit in it the whole time unlike AV turret which you can get off of and move between shots. There would also be a 15m range until the rounds come together, making it not as effective indoors as an AI turret along with its lack of use as cover. That being said, there would be distinct advantages to the design other turrets currently lack. It can better track fast moving vehicles with its rapid fire and higher velocity rather than the wire guide mechanic on the AV turret, it can turn up for better use against air, and it shreds infantry outside the 15m range unless you get sniped which isn't a more significant problem than any other turret. You also get 360 degree's to be able to shoot in for design, imagine a pair of rings on some sort of circular base, then put a chair between the rings and a machine gun on either side of the chair, the base rotates 360 degrees and the rings about 120-135.