Metal and plastic are great, but these DIY cameras are stand-alone pieces of art
Cameras and guns have a long history together. In fact, that relationship deserves its own full-on article, but one of the most fascinating gun cameras has always been the revolver cam. A small camera below the barrel of the gun would take a shot simultaneously as the bullet was fired. As a result, you're left with a photograph of whatever you were shooting at. There's something very Ray Bradbury about the whole thing. It's different from simply attaching an action cam to a gun, which is common practice now. By linking the shutter to the trigger, the gun actually becomes the camera. Of course you could shoot photos without loading bullets into the chamber if you wanted your subject to stick around for future shoots.
Technically, there's already a camera inside of a scanner so this might seem a little like it's cheating. But, creative people have been hacking flat bed scanners to create unique imaging devices for a while, and the results can be pretty interesting. The video above shows how to make a more traditional camera out of a flat bed. Or, you can just try your hand at "scanography" which involves mashing your subject down onto the glass of the scanner and capturing images that way.
The eye-camera has become something of a science fiction trope over the years, but there have been a few individuals who have tried it. Canadian filmmaker Rob Spence, for instance, lost his eye in an accident and replaced it with a low-resolution video camera. The idea has been honed over the years and will likely one day be totally feasible. Not in the near future, of course, but until then it's fun to daydream about adding things like optical zoom to your own eyeballs.