Obviously the GSh-23 or GSh-30 man-portable options would be for the points where the ZSU-23-4 can not access, but these are not many.
Well never say never, but the GSh-30 and GSh-23 don't currently come in man portable versions.
In terms of the GSh-301... it is a very specialised weapon optimised for fighter use with a relatively short barrel life and the real focus on very light weight.
The twin barrel GSh-30 as used on the Hind was not really produced in enormous numbers so I rather doubt it would be used either... I personally would have gone with a GSh-30 twin barrel gun in the BMPT instead of two 2A42s... for that role you want rate of fire and the GSh is rather better than 2 2A42s... but clearly they wanted an Army gun...
The GSh-23 is really only used on the Hind in the current model with it in a chin turret and on paper it should actually be an excellent weapon... compact light ammo with a good HE projectile with a high rate of fire and low recoil, but without a good light mount it is probably going to remain in Air Force stores rather than Army stores.
Of course you do still see it offered for export on things like MTLB upgrades and I would suspect it would be rather useful as a compact but powerful weapon for unmanned land vehicles.
Personally I suspect the 23x115mm ammo might become widespread as a modification of the 14.5x114mm KPV, as there is a version called the KPVB that is adapted to use the 23x115mm ammo... it would be much lower velocity unless they developed APFSDS rounds for it, but the HE round would pack rather more punch... and most of their BTRs had them so they should have a lot.
The main advantege of the GSh-23 and GSh-30 over the ZU-23(-2) is in the massive difference of weight.
The GSh-23 uses much less powerful ammo, but the GSh-30 uses more powerful ammo and both would be similar in weight or heavier than the ZU-23 when on a towed mount.
The key of the issue is which material goes before, and which material can remain longer. Some kind of rank for exits is necessary, and some measures for the complete exhaustion of the material marked as first in line for exit are also necessary in order to avoid rests of these weapons remaining in the reserve.
True but some stock will always be useful, like 203mm shells and 240mm mortar bombs, whereas some are just in storage because they really are obsolete and would only be taken out and used in a very desperate situation... which on its own is a good enough reason to store it.
There is however no point in keeping ammo for which there are few or no weapons that can use it, and equally there is no point in keeping weapons for which there is no ammo or spare parts.
The ZSU-23-4 is not still between the material for fast exhaustion because there is other material less capable, less modern and less powerfull that is very likely to be exhausted before (between them the ZU-23(-2).
I disagree... the ZSU-23-4 uses a chassis and engine that needs to be maintained... so the costs of storage is higher... there is no point in storing them if you can't get parts or they don't make those track links anymore... all of the electronics are largely obsolete, so upgrades might make them cheaper to own, but that has costs too.
In comparison you really just need ammo for the ZU-23-2.
Not all is about age, in the process of exhaustion there is also a technological component. Heavy towed weapons have technological disadvantages that make them to go before than other options. Even making mobile weapons like the ZU-23(-2) mounted in a truck or in a boat, there are design limitations that make them worse option than other options today in the Russian arsenals.
You are not wrong, but the ZU-23-2 by design is simple and rugged... by definition it is going to be easier and cheaper to maintain and operate than an armoured self propelled anti aircraft system like Shilka.
But even between the mobile air defense weapons the ZSU-23-4 seems to be resisting the pass of the time better than other options like the SA-13 Strela-10 and the BRT-D variant with the ZU-23(-2) mounted. I think both will go to exhaustion before than the ZSU-23-4.
That is possibly true but not for the reasons you are suggesting... the Strela-10 and BTR-D and ZU-23 are used by the VDV and the ZSU-23-4 is of no value for a replacement to any of those because it is too heavy for them to operate... even the MTLB based Strela-10 is too heavy to deliver by parachute, so they will need and likely will be getting a new air defence system to replace these systems... and it is called Fowler and will be based on the Pine Surface to Air Missiles... also known as SOSNA...
In comparison to the 5km range Strela-10 and the ZU-23, the 10km range Pine has better range and PK performance with fast but cheap two stage laser beam riding missiles... it is a bit like a very fast two stage Kornet-EM... it is three times faster...
So incase of syria getting the M-30 from Russia they needed artillery to pound enemy and Russia giving them artillery for free hence they choose to give them older equipment and ammo it's a win win situation syria still get artillery to pound enemy Russia gets to clear older stocks.
Ahhh, yes I agree... the D-30s might have been more useful and easier to use, but the Russians could also use such weapons too... the M30 is almost as good and certainly better than nothing.
I hate the word obsolete... the Mosin rifle model 1944 is obsolete on the modern battlefield but it can still kill you, and the ammo is still in use by the country that introduced it... it is not a Soviet round... it is a Russian round... if you know what you are doing and get the right ammo and optics you can kill at ranges the enemy wont even see you from most of the time.
Obviously however there are cases where having 20 or 30 different weapons using the same ammo doing basically the same thing just creates confusion in terms of training and maintenance... and it simply does not make sense to keep things in storage that should be in a museum or scrap metal...
It comes down to a judgement call and of course safety... some ammo can't be stored safely, so when you dispose of the ammo it makes sense to get rid of any weapon or system that uses it... unless that system or weapon is not replaceable so you need to make new ammo...
At the end of the day if Russian armour stores no longer included T-34s and T-10s and T-54/55s, and T-62s, and T-64s, it would not be the end of the world, but if they contained nothing at all then their could be problems...