LB 2-X.
Plenty of ammo, outranges almost everything (which is nice since field guns are not really mobile at all), you can stuff a good 4 guns on an IS motorized platoon and perhaps 5 on a Clan one. 8 or 10 points of pellets is nothing to sneeze at, and it has enough redundancy that you'll still be able to fire a few guns if you take some casualties (unlike, say, a Long Tom or Gauss Rifle platoon). The BV isn't too bad either.