Remember at the end of WWII their air defence was a single barrel 37mm gun, which was replaced with a twin 57mm gun and 14.5m HMGs in various mounts... the towed 57mm guns continued on long after the self propelled vehicle was replaced by the Shilka with 23mm cannon but was replaced in the 70s by the OSA, but they kept the 23mm guns and then replaced them with 30mm guns and missiles on the one platform.
The rate of fire from those smaller calibre cannon meant a pattern of projectiles could be launched through which no normal sized target (ie aircraft) could fly through without getting hit, but small drones and munitions means the rate of fire and number of rounds that need to be fired to assure a hit on even a level flying target becomes impractical.
In terms of hunting animals that move fast like rabbits or ducks the best weapon to use is a shotgun because it offers multiple chances of a hit with each shot... an animal generally does not have thrust vectoring propulsion so if they are moving in a specific direction in the fraction of a second between making the shot and impact of the projectiles they can't move very much in any direction so as long as you have lots of pellets with a nice even spread over a square metre or so the chances are very good you will hit the target with enough pellets to stop it.
Rabbits and ducks are size 3 to size 4 pellets which are a few mms across with about 100 in each shell. Each one will go right through a rabbit or duck sized animal within about 30-20m or so.
Against a human wearing clothes such a load is totally inadequate... our skin is thicker and our flesh is heavier as are our bones... number 3 or 4 shot is more likely to enrage a human than humainely dispatch them.
The problems for the shotgun are the same for the air defence gun... scale up the calibre to have a chance of damaging the target terminally and you reduce the number of rounds per shot (for a cannon heavier projectiles needs bigger rounds and heavier guns that usually don't fire as fast but often fire further.)
The problem is that 000 buckshot might only have 12 projectiles in one 12 gauge shell case... so at 20m if you draw a circle 1m across and randomly put twelve impacts on it that is pretty much what you get with a shotgun... the problem is that you have no control where in that 1m circle those projectiles are going... one shot on a human target you might get one ball bearing through the brain and two through the heart and one through a lung and the rest missing the target completely but that would be a kill... equally the next shot with the same ammo from the same gun at the target standing right next to the first target and you might skim an arm that opens a wound but a trivial wound and all the rest of the projectiles go wide.
WTF am I talking about... well in the 1980s they were testing Gepard (35mm) against a full sized paper target of an Mi-28 at a range of 2km and to be honest the target looked like it had been hit by a shotgun blast from 20m with buckshot... there were about 20 or 30 holes in the target all randomly scattered... if you look on another thread about the 57mm gun mount on a BMP-3 showing firing on the move at a target 2km away with their tight cluster of hits it puts things in perspective... in the 1980s they didn't want air defence guns to be lasers putting all their rounds in a small spot on the target because their target is moving and manouvering so they wanted hits all round the point of aim to compensate for any change in movement by the target between when the gun opens fire and impact time... they didn't want inaccurate guns but they didn't want super accurate either in case the aim point is wrong.
With the new 57mm gun mounts however an airburst ammo they want more accuracy again because it is the fragments that will make up for miss distance... not rate of fire or weight of fire... that is why they only want one barrel... lighter and simpler and cheaper.
It looks like they can fit a 57mm gun mount in to the space of a 30mm gatling gun mount so it will be interesting to see what they do... they generally mount HMGs and rifle calibre machine guns on their ships too so fire power wont be compromised in any event... a 57mm gun firing a single round that hits a small boat sized target would be just as devastating as a burst of gatling fire with a couple of hits and a couple of hundred other rounds splashing all around the target like it was supposed to do...
I think next gen Pantsir will become more stealthy so removing the guns might make a positive difference...