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Gun barrels also have a limited useful life.  NATO built a refurbishing depot in Poland, but it is probably more challenging for Russia.
They are low on rockets also. Being in Ukraine you really feel how less air raids now compared to first months. >

For artillery, it's probably logistic issues more to than shortage. Iirc, it's estimated that they've used 10/17 million shells, and might have gotten more from NK and Belarus.