Vorpal Bullet

You touch the cartridges and combine three into one, charging them with deadly purpose, creating a vorpal bullet. The next you attack you make with the vorpal bullet adds +3 to its attack and damage rolls and pierces all resistances and immunities to the weapon's damage type. On a hit, the creature takes 6d8 additional damage. If this damage reduces a target to zero hit points, it is killed outright by the mortal wound.

Once the cartridge is expended, the spell ends.