Surf is right, except that FF casualties are and were more like 40% - most of them accidental.
Told many years ago that if I ever got to VN I would get looked up and down by troops.
You were simply told that if your platoon sergeant says to you "Sir, I think you should ask for a transfer otherwise on day one, week one, you will become a casualty" - ask for one, the CO would already be warned and waiting to transfer you.