https://www.reuters.com/article/ukraine-crisis-ammunition-csg/restocking-western-ammunition-after-arming-ukraine-will-take-years-producer-idINL8N32R4T9
https://www.businessinsider.com/us-scrambling-for-artillery-shells-ukraine-can-use-against-russia-2023-1
In World War I and II, artillery ammunition wouldn't have been such a problem. The US, Germany, Britain, and Russia eventually developed enough manufacturing capacity to keep their big guns in action. During the Soviet offensive against the Seelow Heights in April 1945, Red Army gunners fired 500,000 shells in 30 minutes.
Unfortunately for the Russian soldiers, their generals followed Soviet military doctrine nearly to the letter, and the Germans had grown used to their tactics. Anticipating a Soviet bombardment, the German generals had pulled most of their men back from the first defensive lines and reduced the number of men in the second lines.
Holy hell