Thanks to a batch of shells from North Korea, Russian forces in Ukraine are flush with artillery ammunition. What gunners don’t have in abundance are howitzer barrels.
www.forbes.com
It does seem counter intuitive that a big metal barrel is hard to make, but modern artillery needs very accurate machining equipment. Russia's existing factories are kitted out with Western gear. You can go back to 50 year old production, but then you have 50 year old accuracy. Accuracy is a lethality multiplier; it's not just more effective, it's several times more effective.
If they could produce enough new artillery barrels, they wouldn't waste their time ripping out barrels from obsolete pieces. Using 50 year old shells from NK will probably accelerate barrel wear, too. The Ukrainians have been seeing more use of shorter range (<155mm) systems as there seems to be a shortage of the longer ranged stuff.
Russia can still make modern artillery barrels. They just haven't shown they can increase how much they make, and the way they're firing guns it's a real concern for them in the middle-ish future. I'll again say that I think they can outlast Ukraine, but they may not like the state of their armed forces at the end of it. It's no good quelling Ukraine if they've nothing left to threaten Kazakhstan with.