If you were to make the photo again, I'd recommend the use of a piece of white or gold posterboard. Have someone hold it near a window and reflect light onto the gun. Or you may try changing the exposure just a tad, perhaps just to overexposed it by 1/3.
If on automatic, SLR cameras try to change the exposure in such a way that everything is neutral gray. In the old days of film, if you took a photo of a black card, the camera adjusts it (by overexposing) to produce a gray image. If you did the same with a white card, it would underexpose in such a way to produce an image of the same gray. I didn't believe it until I tried the experiment on my own.
Of course, that being said, I would have thought the camera should have made this too light. Did you adjust the exposure manually, by telling it to Underexpose -1 or -2 ?
Scratch that last sentence, I just re-read you held shutter opened. You could get similar effects with natural light, and manually adjusting the exposure to your liking...
It's a great shot, nonetheless.