Well yes, but also if it was locked in some space. Because rains happen even in deserts, and sandstorms could do pretty rough sanding, especially to more delicate parts of complex items. When Z found a new ride, it was locked - another was beyond salvaging.
About the ammo going bad, the usual issue is with the primer, which degrades much faster and gives a misfire. Especially if it was in a hot, or humid environment etc. So the gunpowder should be reusable, at least partially - because it also can undergo partial crystallization (may damage the weapon) or jellification (may lodge the bullet).
And yes, blackpowder would lack power (so you would need to rack after each shot) and leave a ton of a nasty shit inside the barrel. Which would be a royal PITA to clean out from the rifled barrel. And I guess that is a main cause of widespread adoption of the barrel rifling after invention of the modern gunpowder.
