So far I've played GitM in Normal and Advanced, so pardon me if Elite is indeed different.
The wiki states (both in the GitM and the Battlepod pages) that Big Iron will shoot 4 times in every phase before getting exposed. By my experience this is wrong. It will shoot 3 times in the 1st phase, 4 times in the 2nd and 5 times in the 3rd one before the shield breaks down. Can anyone confirm this?
Elite is the same phase wise: 3, 4, 5.