Phantom. The 123 body would be bored out, then I would make an extension piece to accommodate the extra length of the 18650. If the diameter of the said cell is not too big of course.
My 18650s measure 18.5mm at the widest point, so the "18" and "17" measurements are rough. 0.5mm does make a difference on whether cells will fit or not. My 17500s are also wider than 17mm...17.xx...don't remember exactly how thick.