How can one capture shots like this where a long, small tube like a gun bore is evenly illuminated along its length?
In this sample the harsh reflections off the breach suggest it was illuminated from the front, but I can't imagine any means of evenly pumping light into a tube no more than .45" in diameter (and as small as .22") while at the same time having the lens centered on the bore's axis, which is necessary to get its full length.
Update: In response to answers so far I did some experiments with single exposures on a comparable barrel. I pasted details as a separate answer below.
Answer
I did some experiments on a 9mm pistol barrel, which is 20% narrower than the .45 used in the reference photo. The barrel I used was potentially 1 inch longer than his (I used a 5.3"; he may have used a standard 5" 1911 or a 4.25" Commander-length).
I used a 300mm lens placed at the minimal focal distance (5 feet) and ran it with minimal aperture (f32-f40). Even with that aperture the focal plane is not quite deep enough to get the barrel in focus from breech to muzzle, suggesting that focus stacking is necessary.
Anyway, using ambient light a single 30s exposure produces this: 
Using two flashlights held as close to the lens barrel as possible produces this 1s exposure: 
Update: I went back to this with a new 90mm macro, illuminated through the chamber from above with the slide open, and stacked three shots taken at f/22 in Photoshop. I'm satisfied that with a lot of careful tuning one could produce something like what was proposed. Here's the result:
Another Update: I got a 35% reflective beam splitter and tested the following setup with a 60mm macro lens:
This does indeed seem to be a viable solution to coaxial illumination, producing the following image at f/20:
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