I've mentioned before that I'm working on a fantasy world where most of the metals we rely on in our world are fairly uncommon outside of a single, inhospitable region of the world. The vast reserves that constitute practically all the world's metal reside in this northern realm and are being hoarded by a technologically advanced isolationist state that uses their monopoly on metal to compensate for their lack of magical aptitude. Ergo, they got to develop useful stuff like metallurgy that lets them make stuff out of steel and bronze while the rest of the world is living in stone buildings and wood huts crafted with magic.
So if every other nation in the world is forced to obtain their metal exclusively through trade with this one technocratic superpower, what does that mean for the prevalence of ballistae and crossbows over cannons and muskets in military conflicts between the other metal-deprived countries? If metal was made artificially scarce enough that most nations had to be very careful when considering the number and distribution of their metal weapons, could that mean you might see wars where cannons and ballistae are being used side-by-side simply because both armies don't have enough metal to completely phase out their wooden ballistae in favor of cannons?
To clarify, these other societies have found ways to adapt to a lack of metal by making armor and weapons out of things like dragon bone and magically crafted glass/obsidian.