This was talked about a while back, but more nuance in actual types/quality of armament would be greatly appreciated. Right now, the game automatically updates each caliber of gun (the "Mark" system) based on year, but doesn't give any choice as to the mark selected -The designer also allows selection for propellant/filling (which should really be separated, by my understanding, but for all intents and purposes work satisfactorily), as well as weight of shell.
The designer does *not* take into account barrel length or other quality differences, for a given caliber of weapon. As you all know, the most obvious example of this difference in real life is the Iowa's 16"/50 Mk 7 gun, versus the South Dakota's and North Carolina's 16"/45 Mk 6. The Mk 7 had increased muzzle velocity and penetration compared to the Mk 6. rendering it a fairly different weapon in practice. However, this is hardly the only example.
When designing new 15" rifles, the French chose a 45-caliber weapon and the British used their old 42-caliber weapon. The Germans, having used a 45-caliber weapon in their WW1-era Bayerns, decided to use a 52-caliber weapon for the Bismarck; likewise, the Italins decided to use a 50-caliber weapon in their Littorios. This lead to the Germans and Italians having muzzle velocities of anywhere from 2,700 to 2,900 f/s, while the British were stuck with roughly 2,500 f/s until they started packing as much propellant as was safe with their "Supercharge" shells, which were never issued to ships with gun elevation beyond 20 degrees. The French attempted to do something similar, but experienced premature bore explosions, and so ceased doing so. Thus, while Italian rifles were markedly less accurate than their British counterparts (which is mostly put down to substandard manufacturing and close barrel spacing, rather than any fault of the gun design itself), they had markedly better muzzle velocity - as did the German guns.
Increasing barrel length, instead of bore size, was thus a viable armaments upgrade, and one that was seen many times from the dawn of the Dreadnought era. The British St. Vincent-class, in 1907, upgraded to a 12"/50 armament, from a 12"/45 armament on the preceding Bellerophons. The American Wyomings did so as well, from the preceding Floridas, while the later New Mexicos upgraded to a 14"/50 rifle from their preceding Pennsylvania-class's 14"/45. The Moltke-class BCs upgraded to an 11"/50 from the VDT's 11"/45. The Japanese Kawachi-class ran into problems because the Japanese couldn't acquire enough 12"/50 guns, and so had to substitute four of the six turrets on each ship for 12"/45 guns. And so on, so forth. That's not even counting how many pre-dreadnoughts operated 12" guns of 40 and even 35 caliber, even just a scant few years before their Dreadnought cousins mounted much larger variants. They certainly served together, and thus can't be merely abstracted as different "marks" of weapon.
While all my examples above are capital-ship weapons, rest assured that these differences continued as the bore size decreases. There's a world of difference between the American 5"/25, 5"/38, and 5"/54 guns, even though all were serving by the end of WW2, and a likewise difference between the Japanese 12.7cm/50 Type 3 and a 12.7cm/40 Type 89. These differences should certainly be simulated in the game, if at all possible, as they were a major part of warship design for the 20th Century.
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Also,while we're on the topic, more denominations of caliber would be nice. Both the French and British used 340mm (13.5") guns on a widespread capital-ship scale, while the German L20e design was set to field 420mm (16.5") guns. The British fielded 4.5, 4.7, and 5.5-inch guns throughout both World Wars, while the French 138mm gun was nigh-omnipresent on it's destroyer fleet. The Japanese used a 140mm gun as well. On a smaller level, many nations used 88mm-90mm naval guns (3.5") as secondary pieces - and, of course, when going even smaller, 40mm/4cm guns were far, far more common than 50mm/5cm pieces. None of these are necessary, of course, but either their implementation or, preferably, the ability to customize, yourself, the bore diameter of a weapon, would be much appreciated.