Does anybody know why Zarqawi was firing blanks?
Was it a safety measure to protect the film crew?
When firing blanks, there is neither sufficient recoil or gas pressure to cycle a recoil or gas operated action. Therefore the weapon must be modified in some way. Usually the method involves a gas restrictor at the muzzle. Even so, the weapon is likely to fail to reload, which is what happened to Zarqawi. He was not expecting that, so his buddy had to cycle the action manually.