Old Idea from CM tank days. Essentially another option for Marines to purchase with techpoints like Foxtrot and Nuke.
Gameplay wise, it would as said before be a tech purchase, with 2 different ways to use it.
It functions as either a ground emplaced weapon, or a cas module similar to rocketpods/GAU.
In ground emplacement it essentially acts as a super mortar, being incredibly slow to operate by yourself, needing multiple marines to use properly. Damage and time to target wise, it sits right between OB and Mortar.
As a CAS module, it can be as simple as just another weapon for a firemission, or more complex requiring actual dropship crew cooperation similar to an AC-130.
Either Variant would be able to use similar but not identical shells to mortar, some suggestions: HE, White Phosphorus, Cluster, Minelayer(timed)
Lorewise it could be a leftover Longstreet LTB cannon(115mm) or a 158mm cannon ripped from a M292 SPA.