"Some grenades can be used against vehicles, gun emplacements (pg 109) and/or Monstrous Creatures (including Flying Monstrous Creatures in Gliding mode). A model can use such a grenade as a Melee weapon, but can only ever make one attack, regardless of the number of Attacks on its profile or any bonuses."
"Melta bombs are cumbersome devices. Melta bombs cannot be used to make a shooting attack."
"Unless used in assaults against vehicles, gun emplacements or Monstrous Creatures, melta bombs have no effect. When used in assaults against vehicles, buildings, gun emplacements or Monstrous Creatures, melta bombs have the following profile: Range - S8 AP1 Armorbane. Unwieldy"