https://en.wikipedia.org/wiki/Close_quarters_combat
<Violence of action
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The defenders often try to stop enemies close to the entry points. The "fatal funnel" is the cone-shaped path leading from the entry where the assaulter is most vulnerable to defenders inside the room. Once operators begin to enter, the defenders may try to keep them from escaping the fatal funnel. The attackers are also vulnerable from the corners closest to the entry point, the first place from which they can be hit from behind as they enter the room. If the first attackers cannot clear the corners and get out of the fatal funnel, allowing those behind to move in and help, the attack can bog down.[7]