Its literally bullets/kill.

Modern urban warfare tactics include a whole lot of shooting where the intent is not to kill the enemy, but to make the enemy hide and reduce their ability to shoot at you.

The army also uses lot of ammo to train and stay competent.

Its similar to how most military pilots clock hundreds of hours flying for every hour of combat they experience.

Bullets/kill is like an extreme version of "game winning scores"/"all shots ever taken in a game or training"

[https://en.wikipedia.org/wiki/Suppressive_fire#:~:text=In%20...]