It's no secret at this point that plasma outperform HEs at the same role, but plasmas are available much later in the game (unless you get very lucky) and require high electronics instead of chemistry, also they have a smaller radius and more expensive components. MkV frags are still worth it since they can outdamage plasmas against squishier targets. In my experience, aside from the early game, HEs are often a waste of AP given the low damage they do. Sure, they do *some* damage against burrower warriors, dreadnoughts, etc but its so little that I may as well have used that 15 AP on something else. These days, many people don't bother getting to 112 chemistry, just 69 for mk4s then switching to plasmas for the lategame. Maybe if HEs had progressively higher armor ignoring percentage? Like, 40%/45%/50%/55%/60%? Or 40/50/60/70/80