How can MEPHISTO 136mm caliber cannon shoot a 150mm ATGM (HOT-2)?
It’s the diameter of the warhead of the missile itself. The launcher is just that, a launcher for fitting and firing a variety of missiles with different warhead sizes. The HOT-2 has a larger warhead diameter, so the game lists it as a larger ‘caliber.’
Hm, I see now. It’s confusing because HOT(136mm) and HOT-2(150mm) have identical in-game models.