Whats the trade-offs
I posing a thread that covers Naval Guns:
* Intermediate size (I'm arbitrarily picking 20mm to 175mm)
* International - US, Britain, Japan, Germany, France, Italy, etc. (CryHavoc101 had a great British thread a while back)
* From the start of WW1 to the end of WW2
* Most importantly - why were those particular sizes chosen. What were their virtues, what were their limitations?
For example: Why did the US more or less standardize on 3" & 5" guns for DE's & DD's (as well as being secondary armament on bigger ships)? Why not 4" or 6" guns?
Fire away!
Path dependency can be a big deal. The US started to go down the 5 inch gun route for its destroyers and primary secondary armament pre-WW1 as the 5/51 was adapted for anti-torpedo boat work on the dreadnoughts. After the war, there is a huge honking stockpile of 5/51 ammunition, as well as 5/25 AA ammunition, so why not design ships to use it instead of spending money to develop a weapon that is just slightly better but Congress won't fund.
More realistically, the question is what set of trade-offs are considered acceptable for a given mission. Let's look at the main armament of a destroyer for the various problems it might be used to solve:
1) Medium anti-ship (sufficient to ruin a destroyer's day, sufficient to sink a merchie) If you want more anti-ship capacity, you'll probably want a longer gun, firing a heavier shell, and probably a wider shell as well. That means fewer rounds stored per meter cubed in the magazine, a bigger firing mount footprint, more crew etc. It still won't be enough gun to really scare away light cruisers and above (an Atlanta or DIDO will avalanche a DD with ROF, a Cleveland will pound away at longer range with heavier shells etc).
So the goal of a medium anti-ship gun is something that has high rate of fire to partially compensate for the lighter shells, a heavy enough shell that can do a fair bit of damage on a few hits, a light enough shell to carry lots of them and for men to be able to reasonably hand-move quickly, accurate, and decent but not amazing range. Those constraints really limit the size. A 40mm or 57mm Bofors is light, accurate, low footprint but it can't hit a target at reasonable range. A 6 inch cruiser gun has a low rate of fire, with non-manually reloadable ammo, and is too big of a gun for a 1750 ton ship. So somewhere in the middle makes sense.
2) Low angle AA --- long range, large bursting charge, variety of fuses, rapid fire, able to quickly skew, able to quickly integrate fire control changes into solution, quick re-fusing etc. Here a long range, high rate of fire gun makes sense, it can be fairly heavily, but ROF probably matters more to breaking up a torpedo bomber attack than the size of the bursting charge.
3) High angle AA --- high rate of fire, high angle, able to easily load at high angles, very fast turning/spinning mount, easy to re-fuse shells. A high angle mount will be significantly more complex and heavier than a low angle anti-ship mount.
These are the three basic jobs a destroyer's main gun battery was expected to perform in the late 30s. The question then is what things will the navy that is buying a DD mandate as have to haves, what things are nice to have, and what things are easily trade-able. And from that set of trade-off decisions, what guns are available whose characteristics fall within the acceptable range. And will Congress/the Treasury fund buying enough of those guns.