How to snap two bones together with a Two Bone IK (Rifle + Left Hand)

I’m currently working on a character that is holding a rifle. But the left hand is always a little bit off and doesn’t stick to the weapon during Aim Offset or other animations.

Since these are two different skeletal meshes, I thought this problem could be fixed with a Two Bone IK setup.
I have set it up so far that I have an Effector that I can move around to move the left hand of the character together with the arm.

But I can’t find a solution to the correct offset of the Effector. It should snap to the bone of the weapon. So I thought giving the Effector the world position of the rifle grip bone should work, but no matter what I try, the position is always off.

Please can someone help me?