You cast this spell on an average sized (spear or smaller) piece of ammunition to be personally used by an ally or yourself (thrown or fired, no siege machines or traps).  The spell fails if it is cast on a piece of ammunition that already has another spell or magical effect (other than a plus to hit and damage) cast upon it with the purpose of enhancing the attack,  or has a similar spell cast on it afterwards. 

When the ammunition is used in an attack,  the spell activates, creating two(2) additional  duplicate projectiles that fly just behind the original. If the original shot hits,  the caster rolls a separate ranged spell attack for each of the additional projectiles to see if they hit their mark.  If the original attack misses or is nullified, the projectiles from the spell also miss.  The damage each additional shot does is the same as the original projectile (dice type, bonuses (both mundane and magical), non-magical extras (poison, fire), etc.) and the damage from each attack is rolled separately by the caster.  At the end of the spells duration all of the projectiles vanish.

This spell creates one additional projectile when you reach 5th level (3 extra projectiles), 11th level (4 extra projectiles), and 17th level (5 extra projectiles).  You have to make a separate  ranged spell attack and damage roll for each projectile.