Just to clarify, what question are you asking?

1: How many/what percentage of Soldiers who experience a direct engagement with the enemy fire their weapons?

2: How many/what percentage of Soldiers who deploy to a combat zone fire their weapons in combat?

3: How many/what percentage of Soldiers fire their weapons in combat during their whole period of service?

1 asks how many Soldiers failed to do their duty.

2 asks how many REMFs there are.

3 asks how many REMFs stay stateside.

do you have a source for this? This does not sound true for any period of history.