“GDP” is irrelevant. In war it is industrial production that is relevant.
Credit default swaps, OnlyFans subscriptions, Uber secondary stock offerings, retail margins on imported Ch!nk crap - none of that is relevant to industrial production of military equipment for war.
The reality - it will take the USA until 2026 to install the new equipment to produce 6000 artillery shells per day.
That is longer than the Manhattan Project !
You are forgetting that the US is not the only “Allied” country producing artillery shells. Rheinmetall alone will boost production to 600,000 rounds in 2024, which is over 1/4 of that 6000 / day. The South Koreans are quite capable too, and... Well, the list is quite long. All that requires money, so, I would not say GDP is ENTIRELY irrelevant.
Moreover, cluster munitions are relevant, since both sides are already using them (Russia first, thank you) in Ukraine, and the US has immense stocks of cluster munitions that never got scrapped because someone “conveniently” neglected to fund the scrapping even though it was stated policy. I read a Human Rights Watch article on this last year, and while head scratching and further web research to try to figure out what really HAS been scrapped turns up a range of quantities, a rough guess would be that the US has around 20 million cluster munitions of various types remaining.
All this misses the point that in (heaven forbid) a direct NATO vs. Russia war, NATO would not likely fight an extended artillery / short range missile duel with Russia in the 1st place. This has ben a sort of proxy war where we’ve largely limited Ukraine to that, plus whatever they can make or turn up in the way of drones, not even supplying ATACMS. I call it Biden’s weak stream of popgun weapons.
Meanwhile Russia has been using almost its entire range of conventional weapons.
Do you seriously think that in a direct NATO vs. Russia war, NATO would refrain from using its other conventional assets?