They did not go to Korea and China because they can't produce ammunition. They went because they consume so much of it...
"The U.S. Army will boost production of 155mm artillery shells more than sixfold to 85,000 a month" - https://www.defensenews.com/digital-show-dailies/2023/03/28/...
Just for comparison...in the Donbas region, Russia is estimated to have fired 40,000–60,000 152mm shells per day...
That is the US Army current monthly production in two days...
The US, again being at peace, is even less motivated to ramp up its production. While the exact numbers aren't currently known, in 1994 the US had 20.4 million artillery rounds stockpiled. It has sent about 2 million of the oldest ones to Ukraine (equivalent to about 6 months of Russian production), and it is also selling large numbers to Israel. The army wants to boost artillery shell production because they always do, but the fact is the US does not currently need to match Russia's production rate, as it is not using shells at nearly the same rate. It's also worth noting that many other nations are likewise producing artillery shells to send to Ukraine. The limitation on western aide to Ukraine has been political will, not insufficient materiel.
And whose electronic countermeasures, by the Pentagon own admission, have been recently defeating some extremely sophisticated US weapons.
Don't underestimate an enemy, who can suffer losses unthinkable for Western Armies, with minimal danger of societal collapse....
"Russia's electronic warfare has repeatedly foiled American precision weapons in Ukraine." - https://www.businessinsider.com/russian-electronic-warfare-s...
"Every day, Russia produces 12,320 artillery shells" - https://bulgarianmilitary.com/2024/05/26/every-day-russia-pr...