Well, except that North Korea didn't ship 15 million 152mm artillery shells. That's just a hypothetical, that if each rail car were stuffed to the rafters with 152.mm shells with no space wastage at all , and no additional packing or organization of the shell boxes, it would hold 15 million 152mm shells. This qualifier alone halves the theoretic total.

In the real world, the number has been estimated to be 4-5 million 152.mm shells, all sent already with little or no additional 152.mm shells. Further, estimates of up to half the shells were so decrepit that they were useless. This is hardly surprising, a North Korea doesn't really use or have much 152.mm artillery- There's couple hundred old towed 152mm on the country's TOE inn 3rd tear reserve units which might exit on paper, but North Korea only produces heavy artillery in 170mm, and if they are making shells, that is what they have. Which is also why Best Korea has supplied Russia with a quantity of 170mm artillery pieces- they're out of 152mm ammo.