Answer by TonyLi

Like @noodle! said, it depends. That said, make the bullet check. :-) If your bullet is moving really fast, even continuous collision might not register. Oliver at Mixamo did a good presentation on this: http://www.youtube.com/watch?v=rS97SG2fbNI -- he uses a script that does some additional checking and also has a cool trace effect. OnCollisionEnter/OnTriggerEnter give you the collider that intersected. Then again, if your bullet is really, really fast, don't bother with a collider at all. Just run a raycast from the gun.