I think Doug may have hit on the answer - limit ammo supply to a D6-worth of turns' firing, or let them fire for twice as many turns at half-effect. Your opponents will want to conserve their arrows to support attacks, or defend against yours, but will be more likely to spread their archers out along their line.
Failing that, I have a lot of crap dice I can let you have cheap, that you can lend them.....
Kein Plan überlebt den ersten Kontakt mit den Würfeln. (No plan survives the first contact with the dice.) Baron Mannshed von Wreckedoften, First Sea Lord of the Bavarian Admiralty.