So, first time posting a thread. I've been GMing a Black Crusade group for a while now, and unfortunately I'm losing a member of the group for the summer. I'm taking the opportunity to switch the rest of the group to Only War for a recreation of the Siege of Vraks, give them some real tank combat against heretics with it evolving into war against the myriad horrors of Chaos (renegade titans, chaos space marines, demon engines, etc).
As part of this, my first scenario is planetfall on Vraks, a little different from the cannon Forgeworld description of the Siege, but something to give them an intro to vehicles and because I have a problem with the first shot of the war starting with already established siege lines. Anyways, I planned to provide the unit (a group of Death Korps of Krieg Grenadiers) with a Centaur Light Assault Vehicle armed with a hunter-killer missile. The idea was to set them an objective (an outpost) that they would need the punch of the hunter-killer missile to break through, and then force them to use the hunter-killer on a surprise renegade Leman Russ Vanquisher (they would be part of a platoon deploying to the battle so they'll have some time to react to the vanquisher as it knocks out other squads before it becomes a real threat to my PCs).
As a result I was looking at what a hunter-killer, and subsequently a hellstrike, would do to a Leman Russ, and I was terribly disappointed. Both pack less punch than a man portable missile launcher armed with a krak round. So my question is, how is it that a large, one shot, vehicle mounted weapon with an availability of Very Rare is less potent than a man-portable, reloadable, Scarce round? As it stands, neither the hunter-killer or the hellstrike have the ability to punch through the forward armor of a Leman Russ, but Black Library fiction is full of examples where these weapons do more.
I'm just curious if this something that anyone else has dealt with, and if maybe there's a more reasonable power level. I don't expect them to one-shot K.O. a battle tank, but more than burnishing the paint would be nice.