"Any sufficiently advanced technology is indistinguishable from magic."
-Arthur C. Clarke
Unlike a typical post-apocalyptic setting, the idea behind Kelvernia is that long-lost technology resurfaces into a fantasy world (somewhat like Expedition to the Barrier Peaks). As a result, there aren't "modern-era" ruins littering the landscape to search for things like grenades and submachine guns. The tech that appears in Kelvernia is "sci fi" tech. Ray guns, force swords, etc. There are two basic reasons for this:
1) It fits the setting: with a highly advanced civilization having left behind technology. Some of it is in disrepair due to the aeons that have passed.
2) The technology is orders of magnitude beyond the current levels of learning in Kelvernia. No one is going to disassemble a ray gun and figure out how to make more.
I plan on treating such artifacts pretty much as magic items. Unlike a ring or a flying carpet, it will require more than just discerning a command word. Player characters will need to make technology rolls to learn how to use it. Failing that, they may need to employ the services of a Technomancer.
Technomancers are sages who study such devices and their origins. Because technological artifacts are so rare, much of their "studies" are simply theories, and frequently wrong. Nevertheless, many do have a knack for figuring out how the gizmos work. Further, sometimes successful technomancers may hire PCs to look for such objects.