@Sig
Lets start at the beginning. For this we have to look at what Chaos gets (as the precursor to "modern" IN): lances and batteries. These are standard weapons for ships then and now. After the Heresy, imo the best way to limit the power of the SM is to limit their mission capability from being to do it all to just doing one thing, which is to deliver marines across the void. Thus they should be hard, armor 6 and shielded (2 for strike cruisers) with capacity for thunderhawks launches. Given limited hull space and energy output per power plant size, the ship's armament of weapons will be diminished. Diminished, rather than altered. Most weapons will be limited to range 30cm, and lances should be sparsely available. I feel this approach allows SM to maximize their primary role and minimize any secondary role that might compete against the IN. I believe this is better than limit what weapons they get. Thus comparing strike cruisers to dauntless and endeavor hull sizes means that if they get armor 6 and 2nd shield, the strike cruiser should be offensively weaker than either.Thus i have no problem with SM getting lances.
I feel this is more flufly than for them to be armed with a new weapon, a weapon with greater punch than weapons available at the time of the heresy, when marine powers were unchecked, and afterward their power limited. Why would the IN agree to give them a new weapon that is better than what the IN have?
I still feel BC is better than both lances and batteries, per hit die rolled. Against necron it is more than 3x likely to inflict criticals. It doesn't matter that necrons can repair on a 4+ because they repair on a 4+ against all 3 weapons. And the more they have to roll to repair, the more they will fail. Against holofield BC is clearly better than lances. Against armor 5 craftworlds and dark eldar capital ships BC hits on a 4+ rather than WB on a 5+. How is this not better than WB? Yes again my calculations are per hit die. Once we see what each weapon is capable of for game balance you can adjust how many each ships should have, and or how many they can bring to bare.
Against high orbital defenses which only have one hit a lance hit vs a BC hit die is the same. Orbital docks, stations, and fortresses have more hits yes, but i just don't see marines fleet mission of delivering SM should encompass taking on orbital stations and fortresses? The primary weapon of SM are the SM, not their ships. Surgical insertion strikes rather than bombardment. Special forces rather than grunts. I just do not see them announcing themselves by taking down an orbital fortress.
In addition to losing BCs, I think the SM should also lose torpedoes. The requirement to contract with AdMech for resupplies i think too onerous for most chapters. Perhaps these should be limited to Dominion fleets.
Balance of the armaments have to consider both unit power and circumstantial availability i understand. But i feel first is decide on unit power then debate its availability.
btw: "balance is achieved by
averaging all situations" ... and i agree