Suppose you have 2 different gunpowder age armies fighting it out in line formation. Both armies have over 30K men. The cavalry or flanking units can do whatever they want, they just didn't engage before the main lines do.
One side (red side) has plenty of ammunition. They fire 4 volleys into the enemy at the range the brown bess fired (was it 100 meters effective?). The first volley does lots of damage, the successive volleys less so. They then launch a bayonet charge on the center and right (enemy left).
The other side (let's call it purple side) is short of ammo and was told to "wait until you see the whites of their eyes" but their powder got wet. Upon the charge, they swear when the realize they can't shoot back.
How many casualties would purple suffer in the first volley (the accurate one) and the next 3 ones (where they used smaller musketballs for faster loading, but this causes bounce)? Let's look at several scinarios.
Scenario 1, both sides are on an open field.
Scenario 2, red is on an open field, purple is just inside a forest. And yes, they still get shot since no one bothered to check their powder until they tried to shoot.
Scenario 3, red is on an open field, purple is in lightly wooded areas with trees and shrubs, but still plenty of straight lines a horse could run through.
Scenario 4, both sides are in lightly wooded areas.
Scenario 5, red is on an open field, purple is on lower lightly wooded areas and they are in a ditch that goes up to their waists.
Obviously purple is going to lose unless their cavalry cooked up some game winning tactic, but that wasn't the question. How many guys will the purple side lose to the red side in the first few shots?