Okay, a close enough look was not taken during the recent correcting of these two in the most recent TacOps errata, for which I take full blame. We've worked out a mix that should provide each with a distinct battlefield role and that also fits the fluff for each shell entry. Under Game Rules, replace the entire entry for each with the following:
Airburst Mortars (p. 373)
“Airburst Mortars are area-effect weapons that are fired at a hex, rather than at a target unit. Airburst Mortars inflict 1 damage point per shell to all targets in the hex, and deliver their damage in 1-point clusters (conventional infantry treat Airburst Mortars as a burst-fire weapon that delivers 1D6 ÷ 2 damage per shell). Only units inside buildings avoid this damage (though the building itself suffers damage to its CF). Each shell of Airburst Mortar ammunition inflicts 2 points of damage in an ammunition explosion (multiplied as normal by the mortar rack’s size and the total number of unfired rounds). Under no circumstances do Airburst Mortars apply the –4 immobile target to-hit modifier, regardless of whether the target of the attack is a hex, is shut down or immobile, and so on.”
Anti-Personnel (AP) Mortars (p. 374)
“Anti-Personnel (AP) Mortars inflict 1D6 burst-fire damage points (rounded up) per shell to conventional infantry units (for example, a full flight of 8 AP Mortars delivers 8D6 damage to a conventional infantry platoon). Against all other units, AP Mortars inflict 1 damage point per shell, applied in 1-point clusters. Each shell of AP Mortar ammunition inflicts 1 point of damage in an ammunition explosion (multiplied as normal by the mortar rack’s size and the total number of unfired rounds).”
To sum up:
Airburst: area-effect, does not affect units in buildings, less infantry damage, more ammo explosion due to "special, heavier shell"
Anti-Personnel: no area effect, affects units in buildings, more infantry damage, less ammo explosion
Note that the ammo notation on p. 416 already matches this, and thus no changes are required there.
Errata will be entered into the TacOps thread to match this. Thanks for your patience.