Folding vehicles works, if you want a weapon that has to be “deployed”.
Say, for a mortar, you make a vehicle frame and call it a “mortar stand”, and then have a “mortar” mounted weapon attached to that. Fold it up for transport, unfold and deploy to fire.
You can also make small towed artillery pieces using non-folding vehicles, since you can attach small vehicles to other ones, now.
Otherwise, if you want to be able to fire it while braced on furniture in general, go with “MOUNTED_GUN” like mentioned above.