I saw the pic of the destroyer in another topic and wonder when the 40mm batteries first appeared on warships.

Was the Swedish-developed Bofors the first effective 40mm? Did other nations use it first?

The 20mm Oerlikon (sp?) ?

Sailors?

I think the US adopted it in late '39 or '40, actually beginning domestic production a few months before it was legal. (alls fair, as they say)
They're were several designs of small cannon used by the US Navy from the civil war forward. I remember seeing a 1 1/2" manually loaded Gatling gun mounted on board a cruiser around the time of the Spanish American war. Remember at that time the boogie man was a fast torpedo boat, not a plane.

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James L. "Jim" Rhiner

Before the 40mm came into use during WWII..the Navy used .50 mg and some sort of 1.1" quad that apparently wasn't very good or very safe to shoot.

By the 1960`s when I was in all of those small guns were gone. The smallest fixed mount I saw was a 3inch/50 cal.
I was told by a WW2 vet the navy learned that the small stuff was ineffective on Kamikazes- they had to be literary blown to pieces and shot out of the air, and not much less than a 5 incher could do that.
My WW2 era destroyer was modernized in 1960 . That`s when the small stuff was removed.

It was developed and quickly spread to other countries, even Germany and Japan captured and used them.

Funny thing is a lot of those smaller guns have reappeared to deal with the small boat threat.

The UK had a 40mm anti-aircraft gun commonly known as the "Multiple Pop-Pom". This weapon had a much lower muzzle velocity than the Bofors but had the advantage of large magazines so a guy did not have to stand there feeding clips all the time. there is a good write up here. QF 2 pounder naval gun - Wikipedia, the free encyclopedia

The Oerliken had a chequered career in that the Royal Navy was very reluctant to adopt it. This gun was rugged but had a slow rate of fire due to its blowback operation. Variants of it appeared as the MG FF in German fighters but it is generally thought that the Hispano and Breda 20mm cannons were better designs. The UK used captured Bredas whenever they could in North Africa and the Med. Of course, the Germans had the MG213 nearly ready for production at the end of WWII and that spawned a large number of weapons in the West such as the British ADEN, French DEFA, US M39 and the Oerlikon KCA.

The 1.1" wasn't particularly dangerous to the gunners, but it had a VERY low rate of fire. The reason was the nature of the action which was designed to minimize stress on deck mountings, and to safely feed and fire the VERY sensitively fused projectiles thought necessary for shooting down fabric covered biplanes.

A very few U. S. vessels were equipped with 1.1" AA guns throughout WWII, but most were given the deep 6 very early on in the war by replacing them with the Bofors 40mm, which was far more effective. The Oerlikon 20mm was also fairly effective as AA if the enemy planes got in close enough.

The 1.1" had a very interesting action with a very mild recoil. The problem was that it was VERY complicated for the result achieved, from a requirement that was essentially obsolete by the time the gun was fielded.

The 1.1" was designed to deal with aircraft which by the time it entered service, only the British were using. The projectile was designed to detonate upon impact with a doped fabric fuselage or wing, an important consideration... if we were attacked by Fairey Swordfish. There was no problem getting a conventional impact fuse to detonate when it struck a Val or a Kate.

I think the 1.1 was a Maxim design; a toggle joint type of operation, very like the MG 08 of WW I, just bigger. It was reliable, but very complicated, heavy, and had a slow rate of fire.

You're thinking about the "Pom Pom" gun, which is basically just the Maxim action scaled up to 37mm and 40mm.

The 1.1" gun was a LOT more complicated than the "Pom Pom", using a system of springs and counterweights to produce a counterrecoiling action to reduce stresses on the ammo and mount. It's a very interesting system, but nowhere near worth the effort to get what was achieved. It also traded reduced recoil forces for a REALLY slow rate of fire. That's why they had to be used on a quadruple mount.

Sweden, Britain, USA, Germany and Japan Bofors 40 mm/56 (1.57") Model 1936
U.S. 40mm Bofors Anti-Aircraft (WWII) - Inert-Ord.Net

These both have some interesting pictures. I learned quite a bit about 40mm projectiles when my Father in Law passed away and I found a box with 10 of them in the garage plus a Japanese Anti Tank Grenade. They had went through 40 years of summer winter in central Indiana. They were guite likely quite unstable.

The Japanese used picric acid (which they called "shimose") as a filling for artillery shells and grenades.

Picric acid is EXTREMELY reactive with metal, forming crystals which are very sensitive to mechanical shock. Famous British weapons writer Ian V. Hogg goes into some detail about his
experiences with unexploded Japanese ordnance in the Doubleday book "Grenades and Mortars".

My aunt's boyfriend was in New Guinea(?) during WWII. He said that they frequently came across boxes of Japanese grenades and mortar shells that had rotted and disentegrated in the humid climate. They would throw a loop of rope to the far side of the ordnance and slowly drag them across the floor or ground. If they didn't explode, they were safe enough to pick up and dispose of.

Most of the small guns where gone by the 60's due to the fact that they where almost useless against jet aircraft. That and at the time, the US military was in the "missals can handle everything" phase.
To the OP question, I think that post the battles of the Coral Sea and Midway, Navy ships where retrofitted with the 20mm and 40mm guns when they came in for major repairs. If I recall, the surviving pre-war carriers lost some of the larger guns to reduce weight for more of the 20 & 40 mm guns.

Here is a Wikipedia link to the USS Enterprise. They list how the armament increased during the war.

https://en.wikipedia.org/wiki/USS_Enterprise_(CV-6)