Snapshot of war from American Soldier

American Soldier: "There is a term called the Fatal Funnel. It's where your team enters a doorway and you bunch up and if someone has a weapon it is very hard to dodge bullets when you are bunched up. Typically a team is killed within seconds.

These doors were not your standard 36inch doorways. So kicking them in would usually break half the door in half and you had to squeeze yourself and gear through the entry way quickly. You wanted to avoid the fatal funnel as best as possible.

*Smash*"

Go read it.
It's not too long.
This is part of why soldiers often don't talk a lot about what they did in the war.