I noticed a weapon bone trembling in a aim pose.
Then I found the cause.
That is setting bone roll perfectly 45° in Blender.
I set the value 45.1°. and trembling stopped.
And the bone was parented to a hand bone but not connected,
Connecting to it also stop trembling.
The animation was baked frame to frame before exported as FBX
so this problem belongs to UE, not the CGI software
