why the Japanese Navy was at a serious mid-range AA disadvantage during the war, as they had no guns analagous to the 40mm so they just had to slap more and more 25mm on their decks. This, plus the lack of proximity -fused shells
Post war the USN developed the 3" automatic gun in response to kamikaze threats.
USN mostly used the Orlikon 20x105 as a morale weapon (gives the cooks and clerks something to do during Air Attack). The IN was similar for their 25mm. Engagement range was only about 1500-2000 yards, or the last 20-25 seconds of aircraft attack (assuming they closed to that close to the ship)
The 40mm Bofors had better reach-out-and-touch distance, and a decent slew rate. And, was an excellent weapon under director control against un-massed air attacks. The problem was that the shells needed insensitive detonators, so they had to hit something substantial in the attacking aircraft--engine, landing gear, main spar, etc.) to detonate. Otherwise, they just left a ±2" hole and whistled on through.
CIWS Phalanx uses the long-proven 20mm Vulcan cannon. There are some installations of 25mm Chain Guns as anti-boat installations, but marine mountings of the chain guns have been problematic.
The jury is also still out on the OTO-Melara 57mm autocannon as a "success." It really does not have the range for Naval engagements (and the automation of the un-manned "turret" has not been a great success).
When Kamikaze massed attacks started in late 1943, there was a scramble to find a way to address that. The VX (proximity fuse) was already under way, but was not going to "miniaturize" to anything smaller than 3" shell. So, USN had a crash development on a rapid-fire dual 3" mount using the proven 3"50 gun. These were designed to fit on a quad 40 base ring, but weight penalties meant they could only be replaced 2 for 3. They entered fleet service in May 1945, and were actually combat tested.
.. and the 105 mm Howitzer shell case that had been re-purposed as an ash tray when I got it...
.. The Howitzer is dated 1943 & the 40mm 1942. It was interesting digging these out again and noticing things like the flash hole on the 40mm is threaded... Looks like a screw in primer must have been used...
... Haven't examined either of these closely in years.
