The fact that both the HE round and the APFSDS round were displayed on the same table is no proof that they are both intended for the Kurganets gun.
But look at them. They both fit the same gun chamber. They are physically the same shape and clearly the same calibre too.
The 57mm S-60 gun round has a bottle shaped shell case that is totally different because more propellent is needed to move the HE round at higher speeds to make it effective. For shooting at aircraft then reasonable velocity is needed. The 23x115mm cannon used in the MiG-21 and MiG-23 and late model Mi-24s the muzzle velocity is about 700m/s so it doesn't need to have enormous velocities... the american equivalent is the 20mm round used in the Vulcan cannon... it uses a subcalibre round with a muzzle velocity of just over 1,000m/s... the Soviet round only moves at 700m/s because its HE capacity is enormous... they could have loaded it with a light subcalibre round like the americans did and their round would be even faster... but they don't want to punch small calibre holes in air targets... they want to launch bombs to blow bits of the target up with each hit and the low velocity means very very high rate of fire so they effectively get a shotgun blast of rounds all round the point of aim making hitting a fast moving target much more likely.
I think the HE round is certainly for that application, while the APFSDS round is for the other gun.
But we know the other gun is based on the 57mm AA round the S-60 fires and is not the same shape as the round on the table.
Seems to be the case indeed. High ballistic cannons are needed against fast and distant targets mainly.
Grenade launchers trade muzzle velocity and range for heavier payloads and bigger bangs... if you were in a BMP-2M and you saw an enemy helicopter approaching you wouldn't think of using your 30mm grenade launcher as your first choice... the higher velocity of the 30 x 165mm cannon makes a lot more sense despite being the same calibre. If the target is jinking and manouvering you would most likely use the Kornet missile to take it down.
LShO-57 is the only reference I have seen. Since it is not a high-ballistics cannon, I assume the 2Axx number does not apply.
The 73mm gun of the BMP and the 100mm rifled gun of the BMP-3 are not high ballistics cannons either but they have 2A designations... 2A28 for the 73mm gun which is just really a closed end SPG-9 recoilless rocket launcher really... if you looked at the rounds they fire they look the same..., and the 100mm gun is the 2A70 (sorry... recently been calling it the 2A80 but just looked it up and it is the 2A70 gun).
I wonder what the rate of fire of this weapon is, possibly way higher than the 2A91, in case the 3ОF91 HE round is used, due to the much reduced loads. That should more than compensate for the reduced ballistic performance in most of the cases.
I would expect the firing rate of the AA gun will be much higher due to the size of the propellent case, while this grenade launcher I would expect the rate of fire with the HE round would be low but the APFSDS round could have a higher rate of fire because of its much larger propellent content and high muzzle velocity.
Look closer to the picture. You could see low pressure HE round 3OF91 with short shell, as it has less powder to fire it. At the bottom you could see high pressure APFSDS round 3UBM21 with regular longer shell for high pressure guns. In front of it is displayed 3BM76 dart from APFSDS round. dart itself is actually to long to be placed in short low pressure shell.
the concept behind the western telescopic ammo is that the round for normal rounds is the same size, so a HE round has a bullet the same size as the AP bullet even though the AP bullet might be a small dart held by a Sabot. The idea with the telescope ammo is that a HE round normally does not need to be super high velocity like an AP round so in terms of capacity or shell volume a HE round might be 50 percent and propellent might be 50 percent, while in the same volume an APFSDS round might be 2% of weight for the dart penetrator and maybe 10 percent more the lightweight Sabot and the remaining 88% can be propellent because an APFSDS round is all about speed.
What the Russians have done here is say... well for our BMPs we had a 100mm grenade launcher with a huge projectile with a big heavy HE round with a small stub propellent case because when you are shooting at ground targets muzzle velocity doesn't matter but the weight of the HE shell does so dial up the weight and dial down the propellent as far as you can with it still getting to target. Their problem wasn't the 100mm though it was the 30mm no longer penetrates enemy IFVs at useful ranges and carrying 20 100mm guided missiles for any armoured target you come across is just not cost effective.
The solution is an old 57mm high velocity gun used by the navy and the army... it has excellent shell capacity for air targets and lots of volume for APFSDS rounds and even guided rounds, but the shells are big and take up room. The competition was a 45mm telescoped round. The ammo is more compact but there really isn't enough space for a guided round.
We also have a 57mm grenade launcher with straight walls and a small stub propellent case and a big long heavy projectile like the 100mm round but for shooting down aircraft you want reasonable velocity because it improves range. The S-60 round would have projectiles maybe a quarter the weight of the 57mm grenade rounds but the grenade rounds will only reach maybe 5-6km while the S-60 rounds could be airbursting target 12km away.
The solution for arming the new BMPs can either be the S-60 type AA gun with big ammo, or the 57mm grenade launcher... but the grenade launcher wont work because you can't fire anti armour rounds and a BMP needs to be able to shoot enemy IFVs with its gun and kill them... the obvious solution is to take the idea of a telescope round and take it a step further. The HE round is a big long projectile with a short stub propellent case. The chamber fits the round but you could use a different shell case with a much longer brass section the length of the HE round that holds the Sabot and the APFSDS round inside it along with propellent that fills the entire area where the HE projectile is on the HE round. It is taking the telescoped case idea and instead of applying it to a gun round applying it to a grenade launcher round.
Imagine a 30mm grenade launcher grenade... technically 30x19mm with a big long projectile and a short stubby propellent case... now imagine replacing that 19mm long propellent case with a much longer case...
Look at this page:
You could easily have a propellent case four times longer... so with basically a 20mm case at the moment that means an 80mm long shell case... perhaps 100mm long propellent case... fill that case up with propellent and put a tiny dart in the front end held in place by a sabot... as long as the mechanism and barrel were designed to take the pressure you could have an APFSDS round.
I know what you are saying... you wouldn't need an APFSDS round on a 30mm grenade launcher... because the 30x165mm gun on a BMP-2 is already more powerful than it would be anyway... but lets say that 57mm grenade is 250mm long... wouldn't you agree that even if the 57mm grenade launcher round had the same dart penetrator as used in the 30mm APFSDS round or perhaps double the weight that the tube 57mm wide and the length of that 57mm grenade and propellent case in propellent is going to give you a very high speed penetrator.
It is not shooting aircraft so its grenade rounds don't need to be fast movers so the bigger HE capacity of the 57mm grenade makes the gun more appealing against the majority of targets. The only problem is that normally grenade rounds are not so useful against hard targets but this APFSDS round means it is.
The ammo is also more compact than the S-60 AA ammo which means more rounds carried in less space.
The S-60 round and more traditional gun make sense for shooting at air targets... less HE with more speed make sense against air targets and the much bigger case capacity and bottle shaped propellent case means even rounds that run the full length of the shell case can have propellent around them to blow them down the barrel whereas the straight sided rounds for the 57mm grenade do not.
except they are both 57mm grenades and can fire old 3OF91, because they look so different judging by their structure shown on the photos...
Keep in mind that even a 125mm shell only has a tiny stub case with a combustable propellent portion.
With a HE round you use the propellent stub which has a small metal stub and the rest is propellent and the projectile is basically a HE bomb... the autoloader loads the bomb and then puts the propellent stub in behind.
With an APFSDS 125mm round you still use the propellent stub... the same stub used to launch the HE round but the APFSDS round needs to move as fast as possible so the APFSDS dart with its sabot to hold it in the barrel has even more propellent around it so it effectively doubles the propellent with a much lighter projectile so the result is the APFSDS round moves at a muzzle velocity of 1.7-1.8km/s and the HE round moves at 900m/s or 0.9km/s.
This 57mm grenade is the equivalent of taking that two piece 125mm round and cutting all the propellent off the propellent stub and cutting it down to the metal stub and attaching a bigger heavier projectile to it. It means instead of moving at 900m/s it might be moving at 300m/s or less.
The target wont care... in fact a 30kg 125mm shell hitting the target at 300m/s will do more damage to the target than a 20kg 125mm shell hitting the target at 900m/s.
The problem is that sometimes targets are armoured and the only way to defeat them is with a penetrator and their solution for the 57mm grenade launcher is to take away that enormous HE projectile that probably weighs 6 or 7kgs that might have had 0.5kg of propellent to blow it down range and replace it with a 800 gramme metal penetrator with a 250 gramme sabot, which means the projectile is just over 1kg and the other 5-6 kgs the round weighs can now be propellent to generate the maximum velocity possible to make it penetrate armour to a decent range...
I think it is very clever.
That straight walled 57mm calibre round lying on the table is the same shape as the HE grenade round standing up... the difference is the size of the propellent charge and case but as with the 125mm gun as long as they are physically the same size these are both one piece rounds that should fit in the same chamber and ammo mag and ammo handling system and fire from a gun designed to fire the round.
In comparison for AA use such a heavy low speed round would be useless against air targets that are moving.
A grenade launcher low velocity HE round can easily still be made to air burst but it would make sense to destroy hovering drones or troops out in the open.... it would be useless for shooting down cruise missiles or incoming hellfires.
A higher velocity round like the S-60 round would be needed for shooting at aircraft all the time.
They could mix the 57mm guns in BMP units like they mixed BMP-1 and BMP-2 vehicles with a HE powerful 73mm gun and the 30mm cannon that was good for other things. The 57mm grenade launcher and 57mm gun compliment each other... the grenade launcher offers a much bigger HE punch, but the gun will deliver a good HE round to targets twice as far away... grenade to 6km and gun to perhaps 12-14km range. The APFSDS round for the grenade launcher would be effective against all sorts of light vehicles. The APFSDS round for the gun will probably be better because it simply has more case capacity for more propellent.
In the past the S-60 had HE rounds and AP rounds designed to be ballistically similar so you could mix different rounds in the same clip and both would fly to the same point of aim. With new ballistic computers they can have higher velocity better penetrating AP rounds as well as slower heavier HE rounds, and also some APHE rounds to punch into ground targets before exploding inside... very effective.
Air burst rounds are ideal for use against aircraft and troops with no top cover and of course small fragile drones so it would make sense if they have a command detonation system for 30mm cannon shells to also use that with 57mm gun rounds and 57mm grenade rounds... the gun rounds would also be used on ships and possibly aircraft too.