They are not, in general, worth it. HP is a two-slot weapon mod that lets you fire HP rounds. They are +2 DV/-1 AP, so +1 DV better than Ex-Ex. However, you get -2 to hit when firing them. If you are so good that you can hit all the time even with -2, then they are on average 1/3 of a DV better. I'm not a big fan - I'd rather use SnS ammo on live targets, and Antitank rounds on drones and vehicles (where you care more about beating their armor than anything else).