There is huge difference in accuracy between a Russian and a western artillery cannon.

At least in 2022 it was commonly estimated that in order to hit a specific target, you need to shoot about ten times as many artillery rounds with a Russian cannon as you would with a western cannon.

Of course, if you don't have a specific target, the accuracy doesn't really matter. The Russian artillery rounds are very useful for carpet bombing a city, but not for any actual military use. And when you're comparing the efficiency of armed forces, actually only the military uses are relevant. For that use, you can use a multiplier of 10. 25 000 western shells have about the same battlefield impact as 250 000 Russian shells.

Then remember that each time you fire a shell, it causes wear and tear in the cannon. That means, the Russian cannons get ten times as much wear and tear as western cannons. Atop them being of a much lower quality. The Russia uses a lot of shells for obliterating Ukrainian cities. Ukraine does not do anything similar. That skews the numbers even more yet.

Just for this contact. There are other suppliers and likely other contracts with Rheinmetall.

Oh definitely, I have no idea how you get actually accurate figures for ratio of artillery shells fired Russia vs. Ukraine but I am sure Russia fires far more (you have to when on the offensive, but also Russia has a massive capacity) but consider the basic logic that if the disparity exists such as it is, that only makes a given increase in artillery availability at a logistical level for Ukraine that much more impactful.

In otherwords the larger apparent disparity in necessary volume of artillery input to achieve a certain degree of battlefield effect (per km of frontline) in favor of Ukraine, the more any increase in artillery supplies to Ukraine translates into a change of the status quo power balance. In this respect, the more Russia is outfiring Ukraine now the worse that makes the calculus look when the tables are more even.

This would be a different conversation if Russia wasn't gaining territory at such a costly rate and at such a slow rate.

On paper I am sure Russia has a dizzying numerical advantage but what does 10,000 155mm shells do for Ukraine vs 10,000 152mm shells for Russia? Russia gets FAR less for those 10,000 shells for a multitude of reasons so I don't know how to relevantly directly compare the numbers.