In fact, this is exactly where this issue is controversial. The 35x228mm high-explosive ammunition charge 112 grams of Hexal PW30, while the 50mm charge is only 190 grams. The charge range of the projectile has increased by 69%, but the weight of the projectile has increased by 181%. Using the Gurney formula to calculate will easily draw the conclusion that the fragmentation velocity is much lower than that of 35x228 (1415m/s VS 1076m/s), and is even lower than 30x165mm rounds, which is obviously unexplainable. What is even more suspicious is that it can be clearly seen from the shooting video that the projectile’s L/D ratio is only about 4, which is much shorter than the abandoned Oerlikon 501MK 50x355mm projectile, but the bullet mass is almost the same.
Although it is possible to increase the weight of the projectile shell by increasing the thickness of the shell, such as the 57x347mm SR ammunition for the Soviet S-60 AA (2800g bullet weight and 154g A-IX-II charge), this is obviously outdated, even Soviet itself had abandoned this idea while designing 30x165mm cartridges for Shipnov 2A42/2A72, and for this reason, it is even more incredible to increase the bullet weight and reduce the muzzle velocity to about 900m/s. A higher initial velocity can significantly shorten the flight time of the projectile and increase the hit rate. As we all know, for projectiles that need to take into account air defense purposes, a higher initial velocity is more important than a heavier projectile when the shape of the warhead is consistent. So my question is here: there are obviously better and been proven by Oerlikon designs: low mass and high muzzle velocity, why do we have to "turn around" to use a worse design?

I have seen no details of the construction of the 50mm HE, but I would expect it to be optimised for airburst. For maximum effectiveness, such a shell might well carry a substantial load of tungsten pellets lining the shell wall, with the HE in the middle. That could put up its weight quite a lot.

Consider the two approaches to fragmentation: a conventional HE/frag like the Bofors with 3P fuzing, which can be set to detonate at a certain point when the target is within the effective radius, and the Oerlikon AHEAD which  carries only a small HE load to burst open the shell; the 35mm AHEAD shell weighs 750g, not 550g. The designers will presumably have been trying out different combinations of HE and fragments (size and number) in order to achieve the most effective all-round compromise for a range of targets.

The 40mm "3P" projectile basically maintains a little heavier weight than the 40mm HE, but Bofors increased the propellant charge to increase the initial velocity from 1005m/s to 1025m/s rather than reduce the muzzle velocity to about 970-980m/s. Although the 35mm AHEAD projectile is heavier, reaching 750 grams, the initial velocity still maintains a high muzzle speed of 1050m/s and make its muzzle energy increase from 380KJ to about 410KJ, which also means that the long-distance trajectory will not be bad. In fact, for modern AA guns, if you want to achieve better killing effects on highly maneuverable targets (such as SDB bombs, anti-radiation missiles, JDAM, small armor-reinforced militarical drones and even MV-22 "Osprey" in full-speed mode), then its muzzle velocity should not be less than 1000m/s. A typical example is PLA newly equipped "625" 6-barrel 25x183mm Gatling SPAAG, which completely eliminated the previous GAU-8-based "LUDUN-2000" 7-barrel 30x165mm Gatling gun in small caliber SPAAG use . One main reason is that the ballistic of the 25x183B is "far better" than that of the 30x165mm projectile, although its muzzle kinetic energy is lower. Therefore, in my opinion, an ideal choice is to reduce the mass of the 50mm projectile to about 1200g, so that the initial velocity can be increased to at least 1030-1050m/s instead of the original only about 900m/s, long-distance trajectory and flight time It can also be guaranteed, which is conducive to increasing the probability of hitting.
However, it is interesting that although China has also introduced the Oerlikon 35mm GDF-003 anti-aircraft gun together with AHEAD, the Chinese-improved AHEAD projectile has a unique feature: the original 152 fragments of Oerlikon have been reduced to 133 (the original 8 layers and 19 per layer are reduced to 7 layers and keep 19 pieces per layer the same), and more Hexal PW30 is added to the vacant layer, which is said to further increase the kinetic energy of a single fragment to have stronger penetration than Oerlikon original version. 
 

In terms of performance against walls and typical cover, are autocannon better with HE or sabots?

Would an antimaterial rifle with tungsten flechette have much use against reaching insurgents behind cover?

The answer is: neither of them will work. Ordinary HE projectiles hardly have the function of prolonged detonation due to the limitation of the fuze. This also leads to the poor penetration efficiency of ordinary HE projectiles on thickened concrete walls that are common in modern urban warfare. Take the 30x165mm 3UOF8 HE as an example, given that the A-670M warhead detonating fuze equipped with it does not have the postponement function (PD-FUZE), it doesn't even have the ability to penetrate the 300mm thick reinforced concrete wall during the test on 2A72. Sounds ridiculous, right? But it does exist. The penetration force of APDS or APFSDS for buildings however is too strong. In many cases, modern urban warfare can even be accurate to the control for a certain toilet-sized room. In this case, the additional penetration power is very likely to cause huge damage to the comrades separated by the wall while killing the established target.So according to the discussion above, adopting APDS/APFSDS to defeat non-armor target in urban warfare is certainly not safe enough.

This is exactly why I am kinda fond of Pele and ALP style rounds for this sort of thing where it more or less uses most of it's energy to dig through the first wall and then "detonates" into the space behind it.

The limited and controlled destructiveness goes where you want it without putting the huge danger spaces beyond your actual target (deeper into the building or into other buildings/spaces behind the area you're trying to hit)

With the sort of tricks we have at our disposal now there's some very interesting things you can do to put the destructive forces more or less exactly into the area you want it to go into without unnecessarily endangering stuff around/behind it.

Another interesting possible path that could be looked into is the explosive liners sometimes used in oil and gas shaped charges for oil exploration and extraction.

In these the actual shaped charge liner penetrates then explodes thereby controlling what it penetrates and limiting it's destructive scope.

Oh. I always envisioned that an antimaterial rifle / MG with APFSDS could fill the role of the failed OICW grenades....with much less cost and weight.

i thought PELE was a variant of APFSDS with an unusual core layout...not some kind of HE loading

Yes, but I think FAPDS might be a better choice. One problem with PELE is that it still belongs to full-caliber ammunition, which also means that the flight resistance will be relatively large. Once the distance between the target and the automatic cannon starts to be extended (for example, in a large city, buildings with a height of more than 400 feet are already can be seen everywhere), the energy storage of full-caliber ammunition will begin to decrease. Moreover, the principle of PELE almost completely relies on the retained energy of the projectile to kill, which is often lethal to projectiles with relatively small kinetic energy (such as .50 BMG) at a long distance. Therefore, it can be seen that in the whole world, the development of the PELE principle projectile only stays at .50 cal and has not been further popularized to any smaller caliber like .338, while FAPDS doesn't have such a problem: the projectile has enough storage capacity, the ballistic trajectory is much flatter, the kinetic energy of any single fragment is enough, and it can even deal with some armored targets like AFV or suicide bomber truck which can be seen everywhere in Syria or Iraq, which is impossible for PELE to achieve. So I think if I were you, I would pick FAPDS to replace HE but keep APFSDS in order to defeat IFV even MBT at a longer range if necessary.

Ah yeah but that's not the niche I was thinking of for what I was talking about. I agree that there's better solutions for that application.

I also was thinking of ALP not Pele itself and ALP actually does utilize a small amount of explosive fill in order to help initiate the PELE style reaction.

The very specific niche I was talking about was for something that's somewhere between 13mm and say 19 mm projectile diameter specifically for smashing through jersey barriers, structural medium compression cinderblock walls or etc barricades ersatz armored technicals/vbied's and etc.

This is stuff we do enough of to be worth having a specific round to deal with it.