When originally written as Guiding the Hunter’s Arrow the spell increased the accuracy of the missile, which under 5th edition Ars magica is somewhat different.
Guiding the Arrow of the Hunter
Rego Herbam 10, R: Sight, D: Momentary, T: Individual
Targeted missile is directed unerringly by the caster. The missile can travel as far as the caster can see, and cannot miss, but is resisted.
(Base 3, +3 Sight)
Guiding the Arrows of the Company
Rego Herbam 20, R: Sight, D: Momentary, T: Group
Targeted missiles are directed unerringly by the caster, affecting many scores of missiles concurrently. The spell cannot miss, but is resisted.
(Base 3, +3 Sight, +2 Group)
Edit – note that these spells do not alter the range or damage of the projectile, they just guide to a target.
Then as an item:
The Hunter’s Wish (ReHe 20) – a bow enhanced to cast Guiding the Arrow of the Hunter when the user utters a command word while firing. This allows the bow to be used normally against magical opponents (who may have MR), and to be deadly accurate against normal opponents. Enchanted as ReHe 10 above, with +10 unlimited uses.
All the new spells for Ars Magica can be found here.