I dreamt about paintball a few nights ago, and the strange thing is there were mortars.
They could be carried instead of a normal gun, but the only way to shoot with them is sitting on the ground. You cannot shoot with it while moving, and you have to pack/unpack before shooting/carrying it to another place (much like trebuchets in Age of Empires 2, if you know what I mean). Its weight also disables you to move fast or jump high.
It shoots several paintballs at the same time, and has a superb accuracy.
A disadvantage may be that it encourages camping.
theres a chance mortars MIGHT (as in, a horridly low chance, but dreamers dream, afterall) but it should use at least 75 or so paintballs a shot, and a long time to even get the mortar working, along with a long time to take down and the inability to move (not a gun, but a secondary weapon, like grenades) but since mortars have a high trajectory, it would probably just end up bouncing off ceilings both visible and invisible, most likely tkilling half a team -.-.
btw, its ammo should probably just be a single, long range paintball.
EDIT: i meant paintgrenade