You know, around the 1300s or so the North Italian urban militias developed a tactic where the crossbowmen were screened by a rank of pavesari, armoured spearmen carrying large "tower" shields. Really a reinvention of the ancient Persian sparabara trick, but hey, it worked.

Rather obviously this wouldn't have been done if the shield-bearers markedly detracted from the firepower efficiency of the crossbowmen (I think they normally kneeled out of the way when at rest).

Also, AFAIK it wasn't all that unusual to have (often rather lightly eqeuipped) horse-archers form up at the rear of shock cavalry while the latter were stuck in and lob arrows over their heads into the rear of the enemy formation. Back in the days when infantrymen commonly carried javelins the rear ranks often tended to "add their weight" to the melee in similar fashion. Not necessarily terribly accurate, but it's not like they really had that much better things to do either.

Just something to think about.

Mind you, for obvious reasons missile troops tended to operate in relatively loose order since most such weapons (particularly slings) need a bit of "elbow room" to use effectively. This likely left enough free space between the men in a rank that at the very least the guys immediately behind them would have had little trouble firing through the gaps, and if for example the front ranks kneeled to fire (dunno if they ever did though) presumably ranks further back could also have clear lines of fire.

Then again, infantry archers normally shot massed plunging fire anyway and slingers were virtually by definition loose-order skirmishers. AFAIK crossbowmen normally operated in either rather shallow formations (probably no more than about four or five deep tops, plus possibly an additional front rank of heavy troopers for close defence and absorbing incoming missiles) or as dispersed skirmishers, so...

Firearms had one advantage over most others in that they took quite little "elbow room" to use properly, which allowed massing the soldiers more closely and hence more focused firepower. 'Course, those guys at least always topped out at a three-rank salvo fire (which also tended to leade them sitting duck since everybody then had to spend the next minute orhalf reloading), but this came to be regarded as worth the sheer shock and firepower.