It looks like you can have smoke rounds as well and you equip it by sacrificing your ammo box. I gather that is how you put smoke rounds in? Dunno have not done smoke yet.
Battlefield 3 Mortar - Battlefield 3
The Mortar in Battlefield 3 is a deployable 60mm mortar that is unlocked and used by the Support class. The mortar can be used with both high explosive shells, and smoke grenade shells as well. The mortar in Battlefield 3 works with the player first deploying it, where after the mini map expands and the player can select a target. The Mortar can be equipped instead of the resupply ammo box.
Also yeah have noticed the kill count is bugged