Mighty Shot
Prerequisites: Ballistic Skill 40
The character knows the weak points in every armour and
material, and has the skill to ensure that his shots land exactly
where they do the most damage. The character adds +2 to
Damage inflicted with a ranged weapon.

How does this work with a Horde?

You get +2 to your damage when compared to the Armour and Toughness to see if the hit damages the horde's magnitude.

Other than that it doesn't do anything against Hordes. It isn't really for use against hordes as most hordes are at a level where you are auto wounding anyway, and as all you need to do is inflict 1 wound for a hit to have an effect on hordes. If you had to roll damage, most of the time the Horde mechanics wouldn't speed up play that much.

Mighty Shot is more for hurting individual targets.

That's what I thought, but doesn't hurt to get that conformation. Thanks!