In my opinion it those are very interesting (but not surprising) ideas Alaskafish.
Prsonally - if I would have to choose one (actually I had to in voting) I would choose standard heavy machine gun. I know that there is already M240, which may be considered to filling this role already (correctly in my opinion), however M240 is more support weapon which is supporting assault squads, and does not require pre-mount. Heavy Machine Gun is on other hand rather defencive weapon, mounted before enemy assault come and needs use more than one person in order to be operate. Also, carrying M240 does NOT make You helpless target for enemy - yes, You are more vulnerable than when carrying PKM or AK47 and so... But not helpless. Having Heavy Machine Gun would render You practically defencless in face of the enemy.
Important note from me - all those fixed weapons should be easy to counter with good sniper. Also I disagree that they should be destroyed when operator is killed. I would say they shoud stay just as vehicles say when it's crew leave it (or is being killd in case of jeep). To destroy it properly one should use grenade, rocket, or just kill all crew (easy to say

) and come and take it apart.
Also, because those weapons would reguire You to have somebody to help, it would be logicall to make them impossible to be carried by those who can't have their squad. Also, possible problem - what if I can have one soldier under my command (and theoretically I can use HMG) but there is no privates that can actually join me? So I can use it, but I can't...
I would recommend making sure that most of bots are privates (more than 50%), that would help maybe. But that is diffrent story, for diffrent theard.
PS. Why do we need to choose only one? All are great

Also, I would advice You to insted of using precise weapons (like M2 Browing) use more generic names related to their roles - like Mountable Machine Gun. Just in case if somebody would like idea of such weapon, but would really don't like M2 Browing (not my case actually).
PS 2. Obviously I am blind, stupid or both as I did not seen that I can choose up to 5 in voting... So my previous PS regarding choosing one is not in place. Second part (that all are great) is still in place
