Still funny how you can learn something totally new about Doom after 25 years of being. Nootrac4571 has finally found the source for the BFG9000 sprites and the exit door textures found in the original Doom games:

For those who aren't aware, it's the toy gun which was photographed to make the BFG sprites and a few textures, like the exit door. Tomorrow I'm planning to take a bunch of measurements and hi-res photos from every angle, and share them for the benefit of folks making HD textures and 3D models.

For more detailed information and some photoshop stuff that makes you notice the origin even more, just check the Doomworld thread.