The Melter can be charged to increase damage, without increasing ammo consumption.
Uncharged, it does regular damage based on the stats on the item card.
Fully charged, all damage is multiplied by ×2, including status effect damage, critical hit damage and Overkill damage.
Status effect damage is only increased by charging or Overkill if the enemy is directly hit by the projectile. Enemies hit only by splash damage and not the projectile itself will only suffer the weapon's base status effect damage.