IK bone does not perfectly follows the target I have no idea why it behaves like that. even when i'm doing animation render, the IK is still miscalculated a lot.

Here's the render result





notice that the hand is slightly moving down from the grip, even though i parent the IK target bone to the front grip. 
the target bone follows the grip perfectly, but the IK bone is just keep glicthed up and down make it seems disconnected from the target.
Open the Blender debug console and make sure Blender is not detecting a cyclical dependency chain in your armature.  IK constraints are the first to freak out when these are present in your model.