He's using them as Field Guns.
(Sorry if it's a bit of a necro situation, but I've been thinking a lot about infantry units recently, so I felt the need to chime in.)
Not quite sure that this is the way to go at all on this subject.
Mortars (Light & Heavy) are found in
Total Warfare (as an optional weapon for IS & Clan battle armor) & in
TechManual (as an
organic support weapon equipping the infantry platoon).
Field Artillery, OTOH,
specifically says it has to be a
single "Field Artillery" or "Artillery Cannon" weapon. Neither of those types of weapons is even mentioned in
TM -- in fact, artillery overall isn't introduced until
TO to begin with -- so I'm going to have to go with, "it's just not happening".
Nor does it need to. The Conventional Infantry Platoon construction rules in
TM already allow you to create a Mortar Infantry Platoon (Foot, Motorized, Jump or Mechanized), with the choice of either 4 or 8 mortars in the platoon.
If you're wanting a mortar unit that feels more like a "tube artillery" unit, you can just make a Field Gun Platoon, equipped with 4 of the 'Mech Mortar-4s from
TacOps (7 tons each = 1 squad can handle a single MM/4), each one getting 1 ton (12 volleys) of ammo.