Below are a rework of gun caliber research date rework, based on mix of current game balance and historical precedence. I will explain the key changes and why they are made.
Guns 14 and up now unlock at mk3 minimum, instead of mk1.
It makes sense in 1911 we do not need to remake 1890 muzzle loaders just to get to new calibers. It will start up to date.
Furthermore, it prevent AI from using outdated design, thus improving AI ship building.
To compensate for skipping mk 1 and 2, 14-18in guns cost 3x as usual to unlock, 4x for 19in, and 5x for 20in.
It also makes sense that increasing calibers takes more effort vs applying new tech to existing calibers.
maintains overall research time needed for the tech tree, thus keeps balance.
In general, 15in and 16in appears earlier, as per this historical appearance dates.
Guns 17-20in appears relatively earlier, but do not unlock at latest mk, unlike 14-16in.
Guns 17-20in historically have been difficult to get right, due to extreme pressure it creates. They start at 1 mk earlier than latest tech.
9in guns nerfed, they no longer unlock extremely early relative to others.
In current game, mk5 9in unlock at 1925, while 10in at 1930. This makes little sense why applying existing tech to 1 inch higher takes 5 years.
mk4 of 15in guns unlocks 5 years later than usual, due to the treaty limitations.
This is the only time delaying tech spread to other existing calibers made sense.