it does seem to drop common from any CWG model with an "X" in its name, i got both mine of an XC and a CX
didnt see it drop off any of the other models and i killed quite a few before reading the posts here saying hunt X's ... also killed a few gnome miners thinking "hey, they mine ore right?" ... no such luck
killed a few Gnome minors too just cuz they were a buncha little whiny brats begging for candy.