*Do you copy?'/Copy that' are the terms used in military radio communication to inform that the message is received and understood.
*Cap is short nickname for captain. Only close friends can refer to their commanding officer like that.
*Bite the bullet' means to do or accept something unpleasant, to be brave in difficult situation. The phrase came from the military because biting a bullet was a common practice for patients, due to a lack of anesthesia.
Jeongguk fought bravely. He will definitely raise a drink in honor of his fallen brothers.
Author's note
Jeongguk was seeing a nightmare full of flashbacks, the events that actually happened in his life and traumatized and hurt him badly.