The bolts do explode in the air if you fire them over a too great distance. It seems that if you fire them in a flat area, they won't reach their mark if you shoot it in a 45 degree angle. It can shoot rather far, but it's by no means a long-range weapon.
Also the constant barrage will give away your position easily and sooner rather than later you'll get a swarm of people trying cash in on the 5-6 easy kills.
I tried the mortar fireteam with JaFaR Ironclad, and came under attack, and I can promise you this much, those were not 5 or 6 easy kills. We had two shields set up and buildings for cover, and it developed into one Heck of a battle.
"One of the bitches actually gave birth while she was attacking, and her puppies joined in on the carnage."
--the awesomeness that is Boatmurdered.