Looks fine in blender, turns into the bottom image after it’s exported. What gives.
https://i.imgur.com/C6XLd9h.png (normal)
https://i.imgur.com/UVSlVmF.png (Broken)
My guess is it’s the weights. Is the gunhead rigid? Did you do automatic weights? If so just parent it as an object maybe.
The old Blender would ask if you want modifiers applied. But I do not think that is it.