There is a bit of an arbitrary dividing line between an autocannon and an autoloader. For example, the German 2cm cannon which formed the primary armament of the Panzer II of the mid 1930s isn't considered an autoloader, it's closer to a magazine-fed heavy machinegun.

I've got a 23-minute video here going from the US autoloader experiments from the early 1940s to the mid 1950s. The 37mm automatic Gun T16 effectively was an autoloading version of the 37mm gun as found on M3 and M5 light tanks. I've two written articles here and here . It was a side-loading system, with trays varying from 1 to 7 round magazines. (In the mechanism held 2 ready in addition to the 1, so a '1-round-magazine' means 'three rounds ready to fire'. As anything bigger than the 1-round meant it intruded into the space used by the commander, and the Army decided that the commander really was someone they couldn't afford to dispense with, which meant it was three-rounds max. However, since a loader could sling the rounds in about as quickly as the tank's gun system took to reacquire a target, there didn't seem to be any point in the increased complexity.

The other early-war 37mm gun the US tried was the one built by American Armaments Corp which used a 5-round curved clip. It was installed on the Marmon Herrington and Tucker vehicles, but as they were all pretty horrible, the vehicles never entered service. (Excepting some of the Marmon Herringtons, due to desperation by the Dutch East Indies and apparently Guatemala). The gun was used on some small naval craft. Suffice to say, the gun was never well regarded by anyone.

The next gun the US tried autoloading was the mid-war 75mm on the T22E1. This set the 'typical' US autoloader design for the next ten years, which involved a couple of cassettes of ammunition, and an elevating loading tray to bring the round up behind the breech for loading. (Again, it's in my video). Similar designs would be proposed for anything up to the M48. This design could dispense with the loader.

The US didn't start looking at revolver autoloaders until the mid 1950s, with the T69, and similar designs were looked at for T54E1, T77, T57, T58. In order to save space, the revolver was not located directly behind the gun (don't forget you probably also wanted to save space for the gun to recoil, fixed-mounted guns were being played with, but not being accepted). As the correct round reached the top of the revolver, a pivoting lifting tray would grab the round from the downward-angled revolver and raise it into line behind the breech for ramming. The thing had a ridiculous rate of fire of a round every 1.8 seconds, meaning the revolver would be empty in about ten seconds. As a result, one still needed to have a loader to feed the revolver, it was purely a matter of rate of fire.

Otherwise, the only related things being built in the 1940s tended to be ram assists, a bit like a warship, on larger calibre guns such as the 155mm on heavy tank T30E1.

Skoda did develop an autoloader for the Panther, the Kwk 44/2, but it never entered service. One for the 8.8/71 for Jagdpanther was also developed, but similarly never entered service. I'm not sure quite what the mechanism was.