There are three meshes:
M1_Abrams is root mesh and Turret is child of M1_Abrams. And Gun is child of Turret.
It seems ok in Blender, and this is my export settings:
But when imported in Unity, Gun part is missing.
But actually Gun is not missing, just transform was messed up. It's rotated and scaled down completely different in Blender.
Why this happens, and how do I fix this? Any advice will very appreciate it.