The turrets placed in the wagons of the Armored Train (Turret Wagons) mod are not loaded with ammunition. The wagon and the turret are separate. This is because the turret is placed in the wagon's inventory grid. That's why there are two ammunition slots. In a sense, the wagon inventory loads the turret. But that doesn't matter, because what's important is that the turret is loaded with ammunition, since that determines the type of ammunition. It doesn't work the other way around. But maybe this information is helpful after all.

https://mods.factorio.com/mod/Armored-train