I am trying to create an entity and bind it to the back of a MiniCopter. The following code works perfectly fine and the flamethrower is in the correct position and rotation up untill the "SetParent()" function. This function rotates the flamethrower and I can no longer change the localPosition, position, localRotation, or rotation but it follows the mini properly. It feels like I've tried everything.

trail = GameManager.server.CreateEntity(FlameEnt, mini.transform.position + new Vector3(0.0f, 0.8f, -1.0f), Quaternion.Euler(0f, 75f, 180f));
if (trail != null) 
{
    trail.SetParent(mini);
    trail.transform.localPosition = new Vector3(0.0f, 0.8f, -1.0f); // doesn't do anything
    trail.transform.localRotation = new Quaternion(0,0,0,0); // doesn't do anything
                    
    trail.Spawn();
    trail.SendNetworkUpdateImmediate(true);        
}

This is probably a client rendering issue. I've seen a similar issue before when patenting a gun to a drone. My understanding is that player weapons have a fixed rotation on one axis, at least from the client's perspective, when the weapon entity is parented. You can change the rotation on only those two axes. Attempting to change on the 3rd will change one of the others. I don't remember any issues with position though. 

Probably a little late to the party here.  But a HeldEntity can act funny when you attach it to something that's not a player.  Try HeldEntity.SetHeld(true) and HeldEntity.UpdateVisibility_GenericVis().  Those methods will affect its localPosition.

I attached a FlameThrower to a MiniCopter but couldn't get the damn flame to shut off.  I set the On and OnFire flags to false but it still just keep shooting fire.  Did you have any luck with that?