As a follow-on to the question How can one capture gun bore shots? I'm wondering how we can get a wide perspective inside a very small space. For example, I like this shot of a .45" barrel with a true macro lens. But if you lengthen the subject barrel or shrink its diameter you'll end up with something more like this shot of a .38" barrel taken with a 300mm.
I'd like to be able to capture a shot of the inside of a longer and narrower tubes from a very close distance: something like a pinhole camera placed 1 inch from the tube end. That would give a striking tunnel perspective from even a very tight tube, and in the limit the full length of the tube would be in focus.
So what are real-world options for approaching that "pinhole" extreme? And what are the physical tradeoffs and limits I have to consider?
Clarification: I don't mean I want to literally get a lens inside the hole. Rather, I want to capture a perspective that looks like it's just outside the hole and magnified enough that even if it's just a quarter-inch diameter tube that quarter inch can almost fill the frame.