IKEA Weaponry: There are a number of these, mostly analogous to their real-world equivalents.  The exception is Eye-in-the-Flame Arms’s Puzzle Pistol, which is a 3D puzzle made from geometric chunks of blue-black diamondoid that can be disassembled from its polyhedral resting form and reassembled into an oddly-shaped cartridge (i.e., non-mass-driver) pistol capable of firing a single preloaded slug.  It’s really more of an executive toy than a practical weapon, but has occasionally been used in practice…

And there’s the way that most weapons are usually assembled from modular components in the first place, sometimes in the field by nanolathes, but those usually don’t come apart again afterwards.