I think "weight of Fire" is a more interesting way to calculate firepower. (weight of projectile * rate of fire per time unit)
But most models for calculating firepower (army manuals for example) take into account probability to hit, the weapon´s /ammunitions´s probability of malfunction and other stuff.
Modern military training involves "putting a lot of lead in the air" during assaults. That´ll make the enemy feel sorry for themselves and not shoot back at you. Of course there is a problem with ammo supply, but there is no real alternative to firing, is there.