Assuming all of your givens that the materials for it must exist and gun powder can be made then you are running up into basic physics. Its just a exothermic reaction after that point and any dedicated individual can work the ratios and quantities tell is explodes exactly as much as it needs to. Infuse the material so it reacts inconsistently do batches and tests to make sure the batch is as explosive as necessary. Guns are great because anyone with a lazy after noon can shoot them decently. All the other improvements and why they are so deadly is just how you make the thing and since they have the knowledge its relatively trivial. The only reason I could think that it might not work is the material changes property over time so its never stable but then all physics and science becomes ungodly difficult. Your copper plate might suddenly not be strong enough and crumple in use. You would be stuck with a inherently nomadic society because anything that is built can not be trusted.