Get a Hitgroup in ENT.PhysicsCollide

Let’s say I have a SWEP that fires an entity using VPhsyics. The entity inflicts damage to entities that it strikes. Simple enough. If the entity hits a player or NPC, it parents itself to the hit entity. Is there a way to get the hitgroup of the struck player/NPC when the entity (Okay, it’s an arrow.) hits it?

I considered doing a trace from the arrow’s pos to the hit entity’s hitpos (via CollisionData), but are there any better solutions? Thanks for any ideas, suggestions, help.