>>USA has 5000+ 155mm howitzers. And 15+ millions shells for them

Seeing as you know so much about US military production and stockpiling, maybe share where those tubes and shells happen to be?

I don't claim to have any inside knowledge, but the US hasn't had an 'artillery first' doctrine since pretty much forever. Maybe you can explain why the US would have 15M shells stockpiled(?) I can tell you that, simply because long-term-stable explosives weren't invented until the 80s, that it's most probable that anything produced before that is not in a stockpile for safety reasons. If I’m not being clear here, I’m saying that any US Cold War era explosives production is NOT in a stockpile.

The tubes could be in storage somewhere as they don't present a hazard, although any made before atmospheric nuclear testing have probably been turned into scientific instruments, but that’s a different discussion.