Kind of really hard to tell what is going on from the angles, but it kind of looks like the shackle is scaling in size. Is that correct?
If so, and if you have an armature modifier on the mesh object, then you might have a loc/rot/scale mismatch problem. The loc/rot/scale of the mesh object, and the armature object (in object mode, obliviously) should match. Best practice is to have the loc & rot = 0 and the scale = 1 on all axis. This information is found at the top of the transforms panel in the 3d view. Ctrl A should be the hotkey to apply/reset the loc/rot/scale.