Well!
(Part 1 of xxx?)
This may come out pretty stream-of-conscious-y. We'll see.
Let's first talk purpose. Why do we have this? You mentioned shore bombardment and fleet command. Area air defense and at least self-defense ASW can be assumed. These tasks can be amply handled by, assuming you're demanding what look like about 18" guns, a Yamato and (to be super extravagant) a CGX-of-doom. Why put it all in one hull? This cramming will have serious issues. We'll talk about that. But let's say that it has to be all in one hull... because.
So, okay, we've dipped our first toe into the waters of the less-than-sane. That's okay. These can be fun. But let's sink deeper and deeper into the depths only when necessary, and only upon great reflection. One wouldn't want to drown.
Let's say we need a battleship-class gun armament, give or take. Well, we're worried about shore bombardment. Even if there were other modern battleships around, no electronics could ever reasonably be armored against more than splinter damage. We presumably need big guns for range and for large HE shells. How big is big, in this case? In WW2, the Nazi 240mm gun was renowned for being a one-hit kill against any target extant. Smaller guns mean more shells and better gun tube life (or a smaller ship, or more guns, or what have you). They also mean less blast impingement on the rest of the ship, and superior ammunition handling. Note blast is potentially a real killer; I have heard that it prevented the fitting of Sea Sparrow aboard the BBs in the 1980s. Whether or not this is true, I've seen it repeated enough that people that know more than I do think it's at least plausible. Meanwhile we
don't want to go the route of WW2-like ammunition handling. The crews required were immense, and people cost a shit-ton of money. I think it's hard to justify anything in excess of 10-12", but I have seen it mused that larger shell diameter can be useful to pack in eg. more smart submunitions and such. So, fine, 18" or thereabouts.
As an aside, why not something like
VGAS? This would dramatically reduce ship impact, cut gun crews to almost nothing, simplify and improve armoring (if you care about that sort of thing), and give the option for all sorts of extravagantly silly blast mitigation systems. Well, maybe we need direct fire for some damned reason. I'm starting to feel a little woozy.
How many gun tubes do we need? Marines care more about the number of independent turrets than they do about the number of barrels, IIRC. You can only lay fire on one target at a time per turret. Assuming modern additives can improve gun tube life sufficiently, a few twin turrets might be just fine. This would probably simplify ammo supply over a triple. Rate of fire isn't really important, at least not compared to staying power (which means total number of rounds in this case). We'd also like all turrets to be well forward. We might need organic aerial spotting, and helos don't mix well with big guns. For that matter, we might shift all of the VLS aft
(let me know when this starts sounding familiar). So maybe something like Nelson/Rodney, with twin turrets so we can push A turret as far forward as possible. We lose 360deg field of fire, but that sort of sucked already (no way we can fire much past 90deg from the turret nominal direction). And you don't care about that for shore bombardment anyway. So, uh, maybe there are archipelagos around that we might need to land on (so we need 360deg). And, uh, maybe we did care about that armoring; a triple turret is the closest to square and is so is apparently the most efficient weight-wise.
I think even Colosseum would concede that we've already made a lot of concessions in the pursuit of
elan. But oh, we've barely just begun. We haven't justified why the BBGMN needs Fleet speed (rather than 'phib), or why we couldn't offload all of those self-defense systems to a nearby destroyer squadron (works for a carrier, after all), etc etc. But fuck it! Enough paragraphs.
Note that if it were up to me, we'd be talking about three twin 240mm turrets forward, B superfiring over A and C. I think. With an aft Iowa-refit configuration, and all radars shoved as far aft as we can manage to help with blast. Nuclear is probably reasonable, as I can shamelessly use the ski-jumps to justify high speed.
Part 1 concludes. Typing continues.