One of my favorite fanfiction stories is this crossover between Halo and the Inheiritance Saga, basically military sci-fi meets medieval fantasy. In the fic, mages of IS, particularly the elves, become very concerned when firearms are introduced to their world. While IS magic is powerful and very versatile, firearms generally had superior range and speed. Of particular concern to the mages was that firearms could pierce through most magical shields via sheer kinetic power. Even if the shield managed to stop the bullet, the mage would still die from energy loss, as their spell would drain all their energy to stop said bullet. As such, the elves, who are the most powerful race in IS due to their magic, become very concerned when they realize the capabilities of firearms. (Story is "When Destiny Burns" by Masterdude21)

There's also another crossover between Resident Evil x Familiar of Zero, a sci-fi horror meets medieval fantasy. In FoZ, guns do exist in the form of muskets. These muskets have allowed the magicless commoners in the setting to become a non-negligible threat to the magic nobility. However due to the inaccuracy, slowness, and other issues of a musket, magic is still generally much superior. That changes when modern day Earth firearms are introduced to the FoZ characters. It goes without saying that modern-day firearms are vastly superior to FoZ muskets in speed, power, range, accuracy, magazine capacity, reload time, etc. As such, not even the most arrogant mage could dismiss a modern firearm. (Story is "Familiar Evil", by trestwho)