I don't know anything about add on making but it doesn't look failed...
Just try and stray away from the abrams design, it looks like one with a 7.62 on top.
The only prob with that coaxial might be there would be no gunner manning it, just a gun firing like the .50 with the gunner looking through the scope.
You can do that though I think, but it requires lotz o scripting.
But don't listen to my word to closely, you are miles ahead of me in add on editing. I haven't even tried....