I was picturing something like this http://images2.wikia.nocookie.net/__cb20061031013425/halo/images/6/6d/Gun_Turret_2.jpg
but obviously not from the halo universe and more modern.
I believe most people refer to it as a tripod. Doesn't necessarily have to be mounted on a tripod but that's mostly where you'll find weapons like this aside from on vehicles. There's also stands that have more than 3 legs or just other kinds of mounts, but they are not really turrets.
In games today people refer to using these types of stationary weapons as 'turret sections' which is probably where you got the idea.This is a turret
(more specifically one found on ships, but you get the idea)
In any case, yes I think I understand what you're trying to say. The turrets, so to speak, can be mounted much like jeeps can, except it puts you inside a pillbox. You would essentially walk up to the door of the pillbox and press the use key to enter it. The problem with pillboxes is that due to the way the game is designed from the isometric perspective it might give the person inside too much of an advantage. Thus they should have a limited view or angle of firing, or you should just make it so these weapons are deployed inside trenches or behind the sandbag wall 'bunkers' that are currently in-game. They would leave you more exposed but give you a high power weapon, and will make it so you cannot move.
For the turrent i was thinking of a mountable sentry gun with a shield that can be broken or grenades tossed over to destroy it.YupLike this
As far as I know nothing like this exists in the real world (EDIT: aside from vehicle mounts).... but if it were to be implemented in a static ground based sort of deal it would suffer the same problems as described above with the pillbox. Except the glass can break, so sustained fire would eventually take it out. I'm not Pasik of course, and I can't really predict how this would actually impact the gameplay...
I must agree that weapons mounted on a tripod stand of some sort would be nice.
Perhaps 'spending' a mortar round to instead set up an emplacement of some sort where you want until 'engineers' are added in? (EDIT: maybe, as you suggested, you would be able to use this method to upgrade a jeep to have a machine gun instead of two rear passengers.)
Also, with pillboxes, maybe enough explosives would make it collapse in on itself, killing the user inside and rendering the 'turret' destroyed for several minutes.
(EDIT 2: I want to point out that I'm not calling anybody stupid or saying they don't know what they're talking about. It's just when somebody refers to a monitor as a computer it kinda bugs me, and the same goes for this kind of thing. I hope you understand what I mean =p )