The standard, Shape Charge ammunition rolls on the cluster table. This makes sense and is balanced, as each cluster does 2 damage but also incorporates the target's movement modifier.
Airburst ammunition, however, is targeted at a hex and has all eight, 1-damage clusters hit everything in the hex. Is there a reason it did not roll on the cluster table? This seems to truly unbalance this weapon, and especially at the listed BV. (3) Mortar 8s would give 24 opportunities at head hits in a single round -- That's quite a bit for a 517 BV vehicle (re.: "Mortar Carrier (Standard)").
