Im sure its likely been mentioned somewhere in here however going through every single post to look for it is a little tedious ANYWAYS the thing I'd like to see intermediate calibers however I'll list through the current ranges you have atm 2-20 inch guns also in the time selections you provide 1890-1945 (both primary and secondaries,mainly secondaries really) aside from the standard 1 inch increments (except the 15" i mentioned for Italy 381mm,doesn't really matter though just 1mm XD) there are the following:
2.2" (57mm quite common with various nations)
2.6" (66mm old Austro-Hungarian and France)
2.8" (70mm only ever used by Spain though from about 1879 to the 1900s)
3.5" (88-90mm,88s exclusive to Germany except 1 Swedish Bofors AA weapon,90mm were widely used by numerous nations)
3.9" (100mm Widely used)
4.1" (105mm,again mainly exclusive to Germany and again Sweden had one as well)
4.4" (113mm exclusive to British lol)
4.7" (120mm gun,widely used)
5.1" (130mm mainly used by Russia and France)
5.2" (133mm,optional really i don't necessarily care about it but im listing anyways,exclusively British)
5.4" (138mm exclusive to France i believe)
5.5" (140mm,mainly Japan)
5.87" (150mm mainly Germany)
6.1" (155mm France and Japan)
7.1" (180mm Spain early on and Russia)
7.50" (190.5mm mainly British)
7.6" (194mm exclusive to France)
7.9" (200mm used by spain from about 1880 to 1910s)
8.3" (210mm,exclusive to Germany)
9.2" (234mm exclusively British)
9.4" (240mm mainly Germany and a French one as well)
10.8" (274mm exclusive to France)
11.1" (283mm mostly used by Germany)
13.5" (343mm older British guns)
15.0" (381mm mainly Italy)
16.25" (412.8mm 1890s British)
And that's all i have,i probably missed some though however there are numerous less than 2 inches but those along with the 2.2" really were used mainly for AA,i hope you guys at least give this a consideration on a side note while ive seen mention of a caliber size sliding adjuster this would be really cool to create your own caliber there's quite a bit of science/numerous calculations amongst other things to find out their ballistics,velocities,penetration,range explosive composition used and shell quality AND probably even more variables so this would very complicated so its not the best of ideas however i hope you give consideration to intermediate calibers,would definitely be cool :D ty for the amazing game keep up the great work!