Ukraine

I get that building factories takes time, but I'm honestly wondering: Given that Russia right now is producing far more artillery shells than the rest of Europe (estimated ≈6x or something), which in turn produce more than the US, and NATO stockpiles are low due to donating to Ukraine: If Russia crosses the NATO border tomorrow, how quickly could Europe get its shell production up to say 3-5 million/year, which is what the current war makes it seem like we'll be needing? It honestly makes me a bit pessimistic to see that it takes us years to build a factory like this. I would like to hope that we're capable of building the tenfold of this in months if shit hits the fan...