In short, US annual artillery production would at best only last for 10 days to two weeks of combat in Ukraine. If the initial estimate of Russian shells fired is over by 50%, it would only extend the artillery supplied for three weeks.
The US is not the only country facing this challenge. In a recent war game involving US, UK and French forces, UK forces exhausted national stockpiles of critical ammunition after eight days.
Such projections are not always accurate, but the broader discussion is interesting throughout.