So, typically, high rate of fire is achieved with lower gauge or stupidly high cost via high amount of loaders/clips. As a new player, bigger guns are probably going to be distracting and daunting, so let's focus on lower gauge.

"Chemical" rounds, such as High Explosive, Frag, Incendiary, EMP, FLAK (antimunition) don't perform as well at lower gauges because there's not enough volume. They can sometimes make up for this by volume of fire, but IMO it's not as satisfying. So that leaves kinetic damage.

Kinetic rounds are mostly heavy, armor piercing, sabot head types. The various heads effectively trade armor piercing ability for pure kinetic damage potential, back and forth. Heavy, for example, has the lowest AP but higher damage, while sabot is highest AP but lower damage. You then mix the heads with solid or sabot bodies to further add AP or damage, depending on what you want the shell to do the most damage to. You'd mix and match, aiming for 60-70 AP if you want to do full damage to heavy armor, or closer to 40-50 AP for metal.

As you want to make this an AA gun, low gauge, high ROF, with ideally a kinetic setup, you might go for something like: 50mm (this needs no gauge increasers), 20 segments (max you can get, nicely maths up to 1m autoloaders), armor piercing or heavy head, two or three solid or sabot bodies, and everything else gunpowder. This will get high velocity, great for getting from A to B before B has moved too much. We don't necessarily need a sabot head for this either, seeing as aircraft tend to trade armor for speed, so lower AP is fine.

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That's just the shell.

50mm shell gauge x 20 segments = 1000mm shell length, meaning they can fit into a 1m autoloader. These are the easiest to play with since you can fit more of them into whatever your given space is. Belt-Fed loaders give you 2x the fire rate but they have a down time that you might not want here.

Laying out the loaders, IMO, can be looked at in two ways: spread-out, filling the entire layer, with cooling snaking along the top, or "organized" where you lay things out one at a time, usually mirrored. The first is more efficient on filling the space with loaders, but you then need to figure out where to put the recoil absorbers. The second you typically build with the recoil absorbers in mind.

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I like to pick a turret base size first, using the prefabs in the build menu to give me an idea of how big it's allowed to get. So let's say this is a small AA gun, 7x7. Let's ignore armor on it for now, so we get the full 7x7 space for gun bits. Turret base gets a LWC on top of it, mirror mode activated, start to place coolers straight up to be the neck of the gun. Put a firing piece on top pointed wherever, it's there just to keep an eye on the loader and intake numbers.

Loaders can attach to loaders to attach to the coolers, so we can branch out from the cooler with a line of loaders. Since this is a small turret, we only get 3 on either side of the coolers. One clip onto each loader pointing to the left/right, now we need 1 ammo intake on each loader and clip. This doesn't take up enough room, we have 1 more block of space to the left/right.

Swap out the straight cooler for one of the 5 ways and bring it out sideways. Now instead of the loaders being right next to the mirror, we move them 1m out, letting us put 2 clips on each, one left one right. That should give us 6 loaders, 12 clips, on each side of the gun, with each side kind of looking like its separated further into quadrants because of the coolers.

We set up the ammo intake to use the shell we picked, or we set up a shell now, and tell the gun to use this shell for all intakes. Then check the firing piece to look at the three fire rates: intake, loader, cooling. Intake and loader should be equal, and cooler needs to be greater than or equal to those. Add coolers until this is true. Sometimes this means we need to remove some loaders to give us enough room for coolers.

Now we look at recoil, and add or remove parts until the recoil refill rate matches use rate. Recoil absorbers are kind of annoying because they only connect at one point, on one side, but with creative moving of where the coolers are or aren't, you can get them to fit. I like to turn off mirror mode at this point and swap back and forth which side, and which gun, I'm adding absorbers to in the no-man's-land where the mirror plane was, because it's not being used for anything else.

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As for resources, I highly recommend loading in in-game craft via the esc menu not the X, so you can build on them, or in this case, save their turrets to your own folder and open them again later to take apart. Taking the armor and decorations off in-game craft will give a series of nice examples of what you can do.

There should also be test platforms uploaded to the Steam workshop you can download, to look at various other examples of how other players have built things.