Sorry for the delay, I was never notified that I had any comments. The I.D. is approximately 0.8 inches. The mount is somewhat flexible and will fit a slightly larger barrel. The difference between the two listings is that this one is customizable via the Shapeways "custom-maker" so you can enter your own text on the bottom.
I realize it has been a while since you left the comment, but I hope this was helpful.