In the case of 300 bow wielding goblins, I would surely apply the rule that a natural 20 is an extra +10 to hit. This would mean the fighter is invulnerable to normal missile weapons. The rule of 20 is not meant to apply to mass combat.
Firing through each other would give the fighter 9/10ths cover after 4 deep which would result in 10 goblin deaths for every hit assuming you were still using a natural 20 hits even when a 40 would not.
A fighter with a duom, great cleave and combat reflexes would kill all 300 goblins in about 6 rounds assuming they all rushed him continuously.
Firing through each other would give the fighter 9/10ths cover after 4 deep which would result in 10 goblin deaths for every hit assuming you were still using a natural 20 hits even when a 40 would not.
A fighter with a duom, great cleave and combat reflexes would kill all 300 goblins in about 6 rounds assuming they all rushed him continuously.