So if an AT (or any) Gun that is being fired on by the 60mm mortar wants to silence it, it targets the squad, half squad, or even a Crew if one is available instead, in the To Hit process?
I apologize, to clarify my question....If an AT Gun wants to take out a 60mm enemy mortar that is harassing the AT Gun, the AT Gun does not fire at the mortar, it targets the MMC that is manning it since the SW mortar cannot be a target itself. Isn't this implied by your original answer?
Yes. The mortar is a SW, which won't have a size TH modifier anyway and which cannot be targeted separately from its owner.
Note that a Gun may not be targeted separately from its owner either, but a possible size TH DRM can apply against it.