When did Cooper shoot Frank in the shoulder?

During the fight scene in Cooper’s office, it cuts to Sarah in the cafeteria for a moment, then back to the fight where Frank suddenly has a bullet wound in his shoulder. When and how did Frank get shot? There were only a couple of seconds between when he was fine and when he had the gunshot wound, and Cooper did not have a gun before or after. Did they cut a scene or is that just a big mistake (a really big mistake considering that the wound plays a critical role in the direction the movie takes from that point).



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